Size-filtered Rotational Spectroscopy of Molecular Aggregates

• Cristobal Perez (University of Valladolid)

Rotational spectroscopy is a powerful technique for determining the precise three-dimensional structure of large and complex molecules. This method provides highly accurate data on molecular geometries and other key parameters. While recent advancements in spectrometer technology have boosted sensitivity and speed, they have also created a new challenge: interpreting the dense and intricate spectra that are now routinely generated. To overcome these issues, we have developed new strategies to "disentangle" these complex spectra. One of the most effective approaches involves using cross-correlation techniques. By examining case studies, primarily focusing on clusters with water, we will demonstrate how these methods simplify the interpretation of complex spectra. These advancements broaden the application of rotational spectroscopy, allowing for deeper investigation into the structure, dynamics, and energy landscapes of molecular clusters and shedding new light on fundamental intermolecular forces.

Detecting Hydrogen-Bound Complexes at Room Temperature - A Joined Experimental and Theoretical Approach

• Casper Vindahl Jensen

Hydrogen bonds are the dominating intermolecular interaction in small neutral molecular clusters. In the atmosphere, the hydrogen bonds promote clustering of water molecules, alcohols and hydroperoxides to form what becomes aerosols and cloud condensation nuclei. Throughout my master’s, PhD and Post. Doc. at the Kjaergaard Group, UCPH, I have worked with detecting, characterizing and quantifying small molecular clusters of alcohols and hydroperoxides with vibrational spectroscopic methods. We take advantage of the redshift and intensity enhancement of the bound OH-stretch and accurately determine the cluster partial pressure by scaling the observed OH-stretching intensity by a calculated oscillator strength. Spectral subtraction of monomer signals leaves only signals from the clusters. We employ high level anharmonic calculations to accurately determine the OH-stretching oscillator strength. Recently we have presented an easy protocol for achieving similarly accurate results using a reduced dimensional VPT2 model implemented in the Gaussian16 package. We have made it possible to determine partial pressures and equilibrium constants at room temperature of a range of atmospherically important cluster species. The room temperature spectrum of the strongly bound water-trimethylamine is unexpectedly structured. In a collaboration with the Suhm Group, GAUG, we were able to measure the complex in three different media and temperatures and we determined that the structure arises as a consequence of coupling between the bright OHb-stretching mode and two other dark combination states of intra- and intermolecular modes. Using room temperature static-, He-jet-expansion- and Ar-cryo-matrix techniques, we were able to tune the states in and out of resonance with each other.

Probing Non-Covalent Interactions in Anisaldehyde Homodimers

• Andres Verde (University of Valladolid)

Non-covalent interactions (NCIs), including hydrogen bonds, π-stacking and tetrel bonds, often act in concert to stabilize molecular aggregates, playing a key role in both supramolecular chemistry and molecular recognition processes. In this work, we investigate the homodimers of *ortho*- and *para*-anisaldehyde, generated in a supersonic expansion and detected using high-resolution rotational spectroscopy. The observed configurations are stabilized by a complex network of NCIs, such as hydrogen bonds, π-stacking and tetrel bonds. Theoretical calculations support the experimental observations and provide deeper insight into the nature and relative contributions of these interactions. A comparative analysis of the homodimers and monohydrated anisaldehyde complexes highlights the distinct roles and nature of the NCIs in each system. These findings contribute to a deeper understanding of how hydrogen bonds cooperate with other non-covalent forces in shaping the structure of weakly bound molecular clusters.

Hydrogen Bonding in Water-Modified Solvate Ionic Liquids Studied by Molecular Dynamics Simulations

• Jule Kristin Philipp (University of Rostock)

Lithium-based batteries rank among the most promising energy storage technologies due to their high energy densities. However, conventional electrolytes often suffer from high flammability and limited electrochemical stability. Solvate ionic liquids (SILs), formed by mixing lithium salts of weakly coordinating anions (e.g., [Li][NTf₂]) and molecular solvents like triglyme, have emerged as safer alternatives. For certain compositions, lithium forms stable 1:1 complexes with triglyme, reducing ion pairing and enhancing ionic transport [1]. To further improve transport properties, we explore the addition of water to this SIL system via molecular dynamics simulations. This approach combines features of SILs and water-in-salt (WIS) electrolytes. We find that water can partially or fully replace triglyme in the lithium coordination shell depending on composition. Water also forms hydrogen bonds with displaced triglyme and [NTf₂]⁻ anions, spatially isolating water molecules and disrupting bulk-like water behavior. This hydrogen-bond-mediated shielding enhances ion mobility while preserving the electrochemical benefits of SILs. However, exceeding a critical water content results in water–water hydrogen bonding and cluster formation, reintroducing bulk water characteristics and narrowing the electrochemical window. Our study demonstrates that hydrogen bonding plays a key role in controlling the structure and transport properties of hybrid electrolytes. By carefully balancing lithium coordination and hydrogen bond interactions, it is possible to design electrolytes that unify the strengths of SILs and WIS systems while mitigating their limitations. References [1] J. K. Philipp, L. Kruse, D. Paschek, R. Ludwig, J. Phys. Chem. B, 2025, 129, 22, 5561–5577.

Halogen Bonds in Rotational Spectroscopy: Mesomeric and Inductive Effects

• Jannes Niels Hasselhorn (Georg-August-Universität Göttingen)

In hydrogen bonding, heavy halogen atoms typically act as acceptors due to high polarizability and moderate electronegativity. By contrast, the situation in halogen bonds is fundamentally different. Here, halogen atoms act as donors due to local electron depletion caused by highly electronegative substituents combined with good polarizability of the halogen atom. This results in the formation of a so called σ-hole, which is an area of positive electrostatic potential. Consequently, there is an inverse correlation between good hydrogen bond donors and good halogen bond donors with respect to electron density. [1] Although there exists some experimental work on halogen bonds using microwave spectroscopy, only a few donor-acceptor combinations have been studied thus far. [1] We have chosen to conduct a systematic study of the substitution patterns of fluoroiodobenzenes in combination with the prominent acceptor pyridine, employing cavity Fourier transform microwave jet spectroscopy. The electric field gradient of the iodine atoms was used as a probe, enabling the quantification of the mesomeric and inductive effects from the fluorine substituents by application of the extended Townes-Dailey model [2,3] combined with intrinsic basis bonding analysis [4] and other theoretical methods. This provides insights into how the strength of the halogen bond donor or hydrogen bond acceptor is affected and therefore how the equilibrium between hydrogen and halogen bonds in iodine-containing systems can be controlled by different substitution patterns. [1] G. Cavallo et al., Chem. Rev. 2016, 116, 478–2601. [2] B.P. Dailey et al., J. Chem. Phys. 1955, 23, 118–123. [3] S. E. Novick, J. Mol. Spectrosc., 2011, 267, 13–18. [4] G. Knizia, J. Chem. Theory Comput., 2013, 9, 4834–4843.

Gas-Phase Vibrational Spectroscopy of Fluoride – Hexafluoroisopropanol Complexes: Solvation-Suppressed Proton Transfer and Isotopic Effects

• Milena Barp (Universität Leipzig)

The selectivity and yield of fluorination reactions depend on a solvent’s ability to moderate the fluorinating reagents’ properties, typically by hydrogen bonding. Recently reported fluorination strategies are particularly efficient when 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) is the reaction solvent.¹ HFIP, an increasingly popular organic solvent, is known for its exceptional hydrogen-bond (HB) donor ability² and HB network formation. However, understanding HFIP’s solvation properties and the formed HB network is far from trivial. Valuable insights can be gained by studying isolated, microsolvated ions in the gas phase, which can be treated by sophisticated computational models. In these studies, we examine the effects of HFIP aggregation on the HB formed in the complexes F⁻(HFIP)_1–3 and F⁻(H₂O/HDO/D₂O). To characterize the first, the complexes are compared to Cl⁻(HFIP)_1,2. For F⁻(HFIP), a strong ionic hydrogen bond with an equally-shared proton motif is observed, which results from a nearly symmetric, single-well potential. The second and third HFIP molecule also directly bind to the fluoride anion, leading to a weakening of the symmetry-equivalent ionic HBs. More detailed information on the nature of the hydrogen bond interaction in these systems is obtained from an energy decomposition analysis.³ The strong hydrogen bond interaction between HFIP and the fluoride anion also influences how HOD binds to F⁻ in mixed-solvent complexes. Fluoride is known to be the only halide anion to bind to the proton rather than to the deuteron in [X⁻, HDO] complexes. However, in the F⁻(HFIP)(HOD) complex, the (HFIP⋯F⁻⋯DOH) motif is identified. [1] N. Shida *et al*., *J. Org. Chem.* **86**, 16128 (2021). [2] M. Barp *et al*., *Chem. Sci.* **16**, 5174 (2025). [3] M. Barp *et al*., *J. Phys. Chem. Lett.*, accepted.

Rotational and Vibrational Spectroscopic Characterisation of Hinokitiol and Its Water Complexes

• Paul Pinillos (University of Bologna)

Hinokitiol, is a naturally occurring tropolone derivative and exhibits a range of biologically relevant activities making it a molecule of considerable interest in medicinal chemistry. Its seven-membered aromatic ring contains a hydroxy ketone moiety that contributes to its potential to form non-covalent interactions, particularly hydrogen bonds, which are key to understanding its mechanism of action in biological environments. The bare hinokitiol was investigated using Free-Jet Absorption Millimetre-Wave (FJ-AMMW) spectroscopy. This high-resolution rotational spectroscopic technique was carried out in the 59.6–78.3 GHz frequency range under supersonic jet-cooled conditions. The recorded spectra revealed the presence of two distinct conformers of hinokitiol. These conformers arise from the rotation of the isopropyl group attached to the aromatic tropolone ring. To explore the microsolvation of hinokitiol and its ability to interact with water molecules, we employed laser-based spectroscopy in combination with time-of-flight mass spectrometry (TOF-MS). The formation of 1:1 and 1:2 hinokitiol–water complexes was investigated using Resonance Enhanced Multiphoton Ionisation (REMPI) and Ion Dip Infrared (IDIR) spectroscopic techniques. All the experimental data was contrasted with DFT calculations, which enabled conformer identification, provided insight into non-covalent interactions, and supported the assignments of the monomer and hydrated complexes.

Competition Between NH…$\pi$ Hydrogen Bonding and Charge Resonance Interactions in Aromatic Heterodimer Radical Cations revealed by IR spectroscopy

• Dashjargal Arildii (Technische Universität Berlin)

Noncovalent interactions between aromatic dimers, including $\pi$-stacking, $\pi$ hydrogen bonding (H-bonding), and cation-$\pi$ interactions play an essential role in biological processes. In radical cations, cation-$\pi$ interactions are stronger than in their neutral counterparts due to the additional electrostatic and inductive effects of the positive charge ($\sim$50 kJ/mol). In charged aromatic dimers, charge resonance (CR), where the positive charge is delocalized over both monomers, is an even stronger force with binding energies of $\ sim$100 kJ/mol. The strength of the CR depends strongly on the differences between ionization energies ($\Delta$IE) of the interacting monomers. Thus, homodimers such as the pyrrole dimer cation (Py2+) are mostly stabilized by the CR, favoring the sandwich structures. In heterodimers, however, the CR is weakened, allowing NH$\cdots\pi$ H-bonding to compete with the CR, favoring T-shaped structures. Herein, we investigate the binding motifs of the pyrrole+-benzene (Py+Bz) and pyrrole+-toluene (Py+Tol) heterodimers, with $\Delta$IE=1.03 and 0.59 eV, respectively, using infrared photodissociation spectroscopy (IRPD) and density functional theory calculations. Analysis of IRPD spectra of mass-selected Py+Bz and Py+Tol, combined with geometric parameters of intermolecular structures, reveals that NH$\cdots\pi$ H-bonding dominates over the CR interaction for both heterodimers (Figure 1). Furthermore, strongly redshifted NH stretch frequencies enable quantitative evaluation of the NH$\cdots\pi$ H-bond strength.

From Small Substituted Heteroaromatic Rings to Highly Flexible Macrocycles: How Intermolecular and Intramolecular Interactions influence Conformation

• Charlotte N. Cummings (King's College London)

Intermolecular and intramolecular non-covalent interactions play a crucial role in the conformational preferences of flexible molecules, which are key in chemical and biochemical processes. Rotational spectroscopy is a powerful technique for the precise determination of molecular structure and when aided by quantum chemical calculations can reveal the intricate interplay of non-covalent interactions present within molecules and complexes. In this talk, we will discuss the flexibility and conformational choices of two different molecular systems. We will show that the isomers *N*-ethylimidazole and 2-ethylimidazole behave differently upon hydration. While *N*-ethylimidazole retains its lowest-energy conformation, optimising hydrogen bonding drives a change in the lowest-energy conformation of 2-ethylimidazole. Secondly, we will focus on the rich conformational landscape of macrocyclic musks, widely used in the perfume industry, and their interactions with odorant receptors. Understanding the relationship between musk conformation and smell is important for the development of new and sustainable musks. We will comment on the non-covalent interactions governing musk conformation and the preliminary results of molecular docking simulations to the human musk receptor will be presented.

Triple Trouble: Hydrogen Bonding at the C≡N Bond

• Alexander Kanzow (Georg-August-Universität Göttingen)

Fourier-Transform Infrared (FTIR) spectroscopy, particularly in combination with supersonic jet cooling, offers precise insight into hydrogen bonds (HBs) in small molecular clusters, providing valuable benchmarks for theory and minimal models of binary mixture behavior. Nitriles play key roles in synthetic and polymer chemistry – as oxidized and essential building blocks or as aprotic solvents such as acetonitrile (MeCN). By forming weak hydrogen bonds, nitriles serve as molecular reporters of how local chemical environments shape noncovalent interactions. Due to their low nitrogen basicity, linear HBs to the lone pair are energetically comparable to side-on interactions at the π-system, where dispersion forces contribute to the binding energy. Besides acetonitrile^[1], this study investigates pivalonitrile (*t*-BuCN) and *tert*-butyl isocyanide (*t*-BuNC) as HB acceptors, with water (H₂O), methanol (MeOH) and *tert*-butyl alcohol (*t*-BuOH) as donors. Notably, the hydrogen bonds to pivalonitrile and *tert*-butyl isocyanide were examined in supersonic jet expansions for the first time. Beyond hetero-dimers, larger hydrogen-bonded clusters such as trimers and tetramers were observed, offering insight into non-additive interactions and microsolvation. Mixed tetramers, in particular, serve as the smallest systems capable of modeling macroscopic miscibility. [1] M. Bödecker, D. Mihrin, M. A. Suhm, R. Wugt Larsen, *J. Phys.Chem. A* **2024**, *128*, 7124–7136.

Improving Spectral Interpretation with Learned Vibrational Coordinates

• Emil Vogt (FS-CFEL-CMI (CFEL-CMI Controlled Molecule Imaging))

Spectral assignment of rovibrational spectra links observed features to labeled energy levels, enabling interpretation through effective Hamiltonians. This labeling relies on identifying coordinates that approximately separate the Hamiltonian into subsystems. For example, in the harmonic approximation, normal coordinates decouple the vibrational Hamiltonian into $3N - 6$ independent modes. However, when anharmonicity is included the assignment becomes less direct, as energy levels no longer correspond directly to individual mode excitations. In general, assigning quantum numbers becomes increasingly difficult as the complexity of the computational model grows. This challenge is especially pronounced for delocalized vibrational states, such as those found in hydrogen-bound systems, where calculating accurate spectra requires advanced models. In such cases, the wavefunctions often show strong mode coupling, reflecting a suboptimal choice of coordinates that do not decouple the vibrational motions. We introduce normalizing-flow vibrational coordinates, a new class of coordinates that can be tailored to a specific system and basis set [1]. Analogous to how spherical coordinates naturally simplify the hydrogen atom problem by embedding physical insight into the coordinate system, normalizing-flow coordinates offload complexity from the basis functions into the coordinates (Figure). This shift improves basis set convergence, interpretability, and spectral assignment. [1] Y. Saleh, Á. Fernández Corral, E. Vogt, A. Iske, J. Küpper, A. Yachmenev, JCTC, 2025, 21(10), 5221-5229

Stepwise Microsolvation Study of Prebiotic Aminoacetonitrile by Rotational Spectroscopy: NH2CH2CN–(H2O)n (n = 1–5, 7)

• Ningjing Jiang (DESY)

Aminoacetonitrile (NH2CH2CN, AAN) is a prebiotically relevant molecule that can hydrolyze to form glycine,the simplest amino acid. It has been detected in interstellar space and in Titan’s atmosphere—an analog of early Earth—where gas-phase microsolvation likely played a critical role, as many prebiotic reactions are believed to have occurred in the gas phase. In this study, we investigate the microsolvation of AAN by one to seven water molecules under isolated gas-phase conditions using broadband rotational spectroscopy in the 2–8 GHz and 8–12 GHz frequency ranges. Hydrates of AAN containing one to five and seven water molecules were experimentally identified, while the assignment of the six-water complex was unsuccessful due to its low dipole moment (strongest µa = 0.5 D). As protonation at either the –NH2 or –CN group is a necessary step in AAN hydrolysis [1], we employed Local Energy Decomposition and N-body analysis [2] to probe site-specific interactions between water molecules and these two functional groups. In parallel, quadrupole coupling constants within the quadrupole principal axes system were employed to semi-experimentally assess the evolving ionic character (ic) of both functional groups. This combined approach reveals how site-selective ionization tendencies develop during stepwise microsolvation. Our results reveal detailed hydrogen-bonding topologies, cooperative interactions, and the evolution of ionic character at each site, providing deeper insight into the chemical behavior of AAN in gas-phase environments.

Spectroscopy Studies on Ices of Astrophysical Relevance

• Christina Tonauer (FS-SMP (Spectroscopy of molecular processes))

To this date, 20 different crystalline and at least 3 distinct amorphous forms of water ice have been discovered. While only hexagonal ice occurs naturally on Earth’s surface, a variety of different ices is present in space, e.g, on icy moons, comets or interstellar dust grains. Water ice on these dust particles acts as a catalyst for the formation of complex organic molecules. Therefore, spectroscopic characterisation of water ices enables astrochemical exploration of icy objects. However, there has been a hiatus of spectroscopic studies on ices after pioneering works from Whalley and co-workers in the 1960s, especially in the near-infrared (NIR) range (10000-4000 cm-1 /1-2.5 µm). The NIR range, however, is of high importance for the exploration of astronomical bodies, since NIR waves pierce through interstellar clouds, revealing objects lying behind. Therefore, we here present [1-3] novel spectroscopic data of 11 crystalline and amorphous water ices in the near-infrared range (see Fig 1). The first overtone of the OH-stretching vibration is a powerful marker for density, porosity as well as hydrogen order, allowing for structural distinction of different ice structures present in space by telescopes such as the James Webb Space Telescope (JWST) or the JUICE mission. **REFERENCES** [1] C. M. Tonauer et al., J. Phys. Chem. A (2021), 125, 1062. [2] C. M. Tonauer et al., Astrophys. J. (2024), 970, 82. [3] C. M. Tonauer et al., PRL (accepted June 2025)

Vibrational Spectroscopy of Monohydrated Radical Cation Complexes

• Ruzica Markovic (University of Copenhagen)

In neutral bimolecular complexes, the hydrogen bond is the strongest possible intermolecular non-covalent interaction. It is well-studied with techniques such as infrared spectroscopy, due to the typical redshift in wavenumber and enhancement in the intensity of the bonded OH-stretch fundamental transition. In the present work, the monohydrated radical cation complexes (H$_2$O-X)+ are considered with X = Ar, N$_2$, CO$_2$ and N$_2$O. The vibrational spectra are calculated using second-order vibrational perturbation theory (VPT2) and a more advanced local mode model (LM). The VPT2 and the LM models predict redshifts of the bonded OH-stretch in hydrogen-bonded charged complexes that are an order of magnitude larger than their neutral counterparts. This can be attributed to the increased binding energies in the charged complexes. The transition wavenumber of the bonded OH-stretch in (H$_2$O-Ar)$^+$ predicted from our calculations agrees with the experimentally observed transition. In the sequence of the binding partners from Ar to N$_2$O, the calculated redshifts increase, in agreement with the increase in the binding energies. In charged radical complexes, an additional binding motif occurs through the hemi bond, which has a bond strength comparable to that of the hydrogen bond. To aid the interpretation of the experimental spectra, it is important to calculate the OH-stretch transition energies accurately for both the strongly hydrogen-bonded and hemi-bonded isomers. Such considerations are especially important because of the complicated nature of the spectra obtained for hydrated radical cation complexes, owing to the limitations in techniques such as photodissociation spectroscopy.

Probing Binding-Site Preferences in Propiolic Acid Complexes with Water and H2S inside Helium Nanodroplets

• Arghya Chakraborty (Post-Doctoral Fellow)

Binary molecular complexes serve as model systems for probing hydrogen bond (HB) interactions and are studied using molecular beam and matrix isolation techniques, which favor the isolation of thermodynamically stable structures. In contrast, the helium nanodroplet (HND) technique, owing to its ultracold environment (0.4 K) and efficient energy dissipation, enables the kinetic trapping of higher-energy isomers, revealing a wider variety of hydrogen bonded motifs that are otherwise inaccessible. Interaction of organic acids with small molecules are crucial in biology and atmospheric chemistry. So, we investigated the HB site preferences in 1:1 complexes of propiolic acid (HC≡C–COOH, PA) with D2O and H2S inside HNDs. Mass-selective vibrational spectra recorded in the C=O and C≡C stretching regions, complemented by MP2-computed harmonic IR spectra, confirmed the isolation of the cis-PA. Complexation with D₂O resulted in the formation of three isomers of the cis-PA···D2O. The dominant spectral features correspond to a kinetically trapped structure stabilized via a ≡C–H···OD₂ HB. Contrarily, PA···H₂S forms exclusively the global minimum structure, aggregated by two hydrogen bonds with the COOH moiety. These findings demonstrate the balance of the dipole–dipole and higher order interactions in steering aggregation dynamics at 0.4 K. The polar D2O (1.85 D) and PA (1.59 D) promotes directional association, whereas the weaker dipole of H2S (0.97 D) favors the global minimum structure.

Quantifying Hydrogen Bonding Interactions in Gas Phase Model Systems Using Ion Vibrational Spectroscopy

• Knut R. Asmis (Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig)

Recent advances in the gas phase vibrational spectroscopy of hydrogen-bonded ion-molecule complexes in the context of (i) anion microsolvation[1-3] as well as (ii) enantiomeric excess determination[4] are described. The use of cryogenic ion traps in combination with widely wavelength-tunable IR lasers in the context of vibrational action spectroscopy has allowed for detailed molecular-level insights into the nature of hydrogen bond interactions, in particular, when combined with electronic structure calculations.[5] Advances and challenges in the interpretation of infrared photodissociation spectra, in particular, the importance of considering anharmonic as well as entropic effects, correlating vibrational frequency shifts with molecular properties (e.g. hydrogen-bond acceptor/donor strengths) as well as quantifying solution phase properties through gas phase measurements will be discussed. References [1] M. Barp, F. Kreuter, J. Jin, R. Tonner-Zech, K.R. Asmis, J. Phys. Chem. Lett. in press (2025). 10.1021/acs.jpclett.5c00953 [2] M. Barp, F. Kreuter, Q.-R. Huang, J. Jin, F.E. Ninov, J.-L. Kuo, R. Tonner-Zech, K.R. Asmis, Chem. Sci. 16, 5174 (2025). 10.1039/D4SC08456J [3] Y. Ni, J. Lebelt, M. Barp, F. Kreuter, H. Buttkus, J. Jin, M. Kretzschmar, R. Tonner-Zech, K.R. Asmis, T. Gulder, Ang. Chem. Int. Ed. 64 e202417889, 1 (2025). 10.1002/anie.202417889 [4] S. Schmahl, F. Horn, J. Jin, H. Westphal, D. Belder, K. R. Asmis, ChemPhysChem 25, e202300975 (2024). 10.1002/CPHC.202300975 [5] N. Heine and K.R. Asmis, Int. Rev. Phys. Chem. 34, 1 (2015). 10.1080/0144235X.2014.979659

Photoreactivity of PAHs as a function of the local hydrogen bonding environment, from isolated molecules to clusters and ices.

• Jenny Noble (CNRS)

The photoreactivity of aromatic molecules is key to the physics and chemistry in a wide range of environments. In the interstellar medium, polycyclic aromatic hydrocarbons are believed to account for a significant fraction of the carbon budget, and their propensity to undergo photodetachment, photofragmentation or emission (e.g. recurrent fluorescence) following photoexcitation will determine local physical and chemical conditions in diffuse environments. In media dominated by hydrogen bonding (solutions, clusters, ices), the photoreactivity of aromatics will be modified. For example, due to the ultrafast dynamics in their electronic excited states, small aromatics play the role of UV photoprotectors in the human body. Due to their ubiquity, and the wide range of relaxation timescales they demonstrate, aromatics are an interesting target for studying molecular dynamics in various hydrogen bonding environments. In this talk, I will review some recent experimental and theoretical studies of the photophysics of small aromatic molecules spanning experimental conditions from the gas phase to the solid phase and probing the role of parameters such as charge state, heteroatoms and functional groups in driving the UV-induced reactivity within these species.

A PHOTOPHYSICAL INVESTIGATION OF LANTHANIDE COMPLEXES BASED ON LEVOFLOXACIN

• María Mar Quesada-Moreno (Universidad de Jaén)

Antibiotics hold potential for future applications in cancer treatment, with certain types of tumors being targeted. Levofloxacin is a notable example that has been widely investigated for its anti-tumor activity. To improve its therapeutic performance, it has been employed as a ligand in coordination complexes with metal ions. Levofloxacin and its derivative compounds have gained significant attention due to their luminescent properties. Thus, this organic compound could be used as antenna in coordination complexes to combine its antibiotic properties with the superior luminescence properties of lanthanide ions, which make levofloxacin complexes particularly appealing for probing biological environments. This work is focused on studying electronic and photophysical properties of levofloxacin and its coordination compounds with the metallic ion Zn(II) and the lanthanide ions Tb(III) and Eu(III) experimentally and theoretically. In the solid-state, hydrogen bonds between levofloxacin molecules from different units stabilize the overall structure. In solution and solid state, the emission spectra of levofloxacin and its Zn(II) coordination complex show the same profile. However, the emission spectra of its Tb(III) and Eu(III) coordination complexes in both phases display the evident narrow lanthanide emission spectra, indicative of a clear antenna effect. This is also clearly observed when the complexes form in a cellular environment, confirming the feasibility of studying their localization and monitoring their activity as drugs using bioimaging techniques. We also report a detailed photophysical and Time-Dependent Density Functional Theory (TD-DFT) computational investigation that unveils antenna effect’s performance and the different sensitization pathways involved.

HCOOH-NH3-H2O, a prototypical acid-base-water cluster revealed by rotational spectroscopy

• Fan Xie (FS-SMP (Spectroscopy of molecular processes))

The ternary complex HCOOH-NH3-H2O, serving as a prototypical acid-base-water cluster relevant to atmospheric nucleation processes, was investigated using rotational spectroscopy in conjunction with theoretical calculations. The complex adopts an effective Cs symmetry with two tunneling motions: NH3 internal rotation and free -OH wagging. Ammonia acts as the proton acceptor to formic acid, resulting in a transitionary chemical bond with both hydrogen bond and ionic bond characteristics. The measured 14N nuclear quadrupole coupling constants indicate that the ionicity of the ternary complex (27%) is higher than that of binary complexes such as HX-NH3, (X = -COOH, F, Cl, Br, I). These results suggest that the inclusion of a single water molecule significantly enhances proton and electron transfer between the acid and base molecules by reinforcing the hydrogen bond network. This model could serve as a prototype of acid-base-water ternary aggregates, offering valuable insights into the gas-to-particle phase transition mechanisms.

To be or not to be hydrogen-bonded: Supercritical water and (H2S)2

• Martina Havenith (Department of Physical Chemistry II, Ruhr University Bochum, Germany)

Water also serves as the prototype for directional hydrogen bonding at ambient conditions. However, the question of whether supercritical water is still hydrogen-bonded or how water molecules interact in the supercritical regime is a matter of controversial discussions. We present terahertz (THz) spectra, which directly probe the intermolecular interactions of water under these extreme conditions [*Sci. Adv.* 11, eadp8614, 2025]. While we spectroscopically detect the liquid-gas phase transition just below the critical point, THz spectra of the high-temperature gas phase are indistinguishable from those of supercritical water at the same density. The accompanying ab initio simulations provide the molecular underpinnings: The water-water contacts at supercritical conditions are essentially orientationally random. Furthermore, we examine hydrogen bonding in the H2S dimer, in comparison with the well-studied water dimer, in unprecedented detail [*Nat. Commun.* 15, 9540, 2024]. We record a mass-selected IR spectrum of the H2S dimer in superfluid helium nanodroplets. We are able to resolve a rotational substructure in each of the three distinct bands and, based on it, assign these to vibration-rotation-tunneling transitions of a single intramolecular vibration. With the use of high-level potential and dipole-moment surfaces, we compute the vibration-rotation-tunneling dynamics and far-infrared spectrum with rigorous quantum methods. We show that the intermolecular modes in the H2S dimer are substantially more delocalized and more strongly mixed than in the water dimer. The less directional nature of the hydrogen bonding can be quantified in terms of weaker electrostatic and more important dispersion interactions. The present study reconciles all previous spectroscopic data and serves as a sensitive test for the potential and dipole-moment surfaces.

Kicking off the second HyDRA blind challenge on microhydration: Challenge details and first test set announcement

• Margarethe Bödecker (Georg-August Universität Göttingen)

The field of gas phase spectroscopy benefits from a robust interplay between quantum chemistry and experimental techniques. To facilitate the advancement of systematically enhanced quantum chemical predictions in vibrational spectroscopy and solvation science, we have helped to organize the first HyDRA (Hydrate Donor Red Shift Anticipation) blind challenge during the 2021/22 academic year.[1] Within this challenge, ten cold 1:1 complexes of organic molecules with water were prepared in a vacuum and examined by infrared and Raman spectroscopy for the first time. The fundamental OH stretching vibration of the hydrogen-bonded water molecule in these complexes was measured and kept secret until ten interested theoretical research groups worldwide had made their predictions in spring 2022. The detection of a robust anharmonic resonance in some of these water complexes rendered this competition not only interesting for scaled harmonic predictions, but also for more elaborate anharmonic treatments. Now, in the sequel challenge, theoreticians are provided with a seven-fold larger training set compared to the last challenge. However, the theoretical models are now further challenged by the request to predict stronger hydrogen bonds and dihydrates. This contribution summarizes the results of the first HyDRA challenge [2] and launches the second round of the HyDRA blind challenge by announcing for the first time the molecular test set systems which define HyDRA II. References: [1] Taija L. Fischer, Margarethe Bödecker, Anne Zehnacker-Rentien, Ricardo A. Mata, Martin A. Suhm, Phys. Chem. Chem. Phys., **2022**, *24*, 11442. [2] Taija L. Fischer *et al*, Phys. Chem. Chem. Phys., **2023**, *25*, 22089-22102.

L-DOPA Autoxidation is Controlled by Water Proteolysis

• Janez Mavri (National Institute of Chemistry)

L-DOPA (tyrosine with an additional OH group) is dopamine (a monoaminergic neurotransmitter) precursor and is used in pharmacological treatment of Parkinson disease. Both dopamine and L-DOPA are unstable and beside MAO B enzyme catalysed dopamine decomposition [1] they can enter autoxidation reactions giving rise to hydrogen peroxide [2][3]. Moreover, L-DOPA is randomly built to proteins instead of aromatic amino acids where it is also autoxidized. The rate limiting step of the intramolecular Michael addition is water proteolysis. A similar rate-limiting step has been observed in carbonic anhydrase II [4] and staphylococcal nuclease [5]. Using the Empirical Valence Bond (EVB) method, we computed the free energy profiles for the reaction of L-DOPA incorporated into MAO A, replacing Tyr407 by considering full enzyme and water dimensionality. Critical step for EVB is evaluation of the experimental free energy profile for water proteolysis in bulk water and early experimental work of Eigen and De Maeyer [6] was critically revisited. The calculated barrier of 33.93 kcal mol-1 is 6.38 kcal mol-1 higher than the experimental barrier of 27.55 kcal mol-1 for L-DOPA in aqueous solution [3][7]. [1] PROTEINS:Structure, Function,and Bioinformatics,82 (2014) 3347-3355. [2] N. Umek, B. Geršak, N. Vintar, M. Šoštarič, J. Mavri, (2018), , Front. Mol. NeuroSci. 11, 467. [3] A. Prah, J. Mavri, J. Phys. Chem. B 128 (2024) 8355−836. [4] J. Åqvist, A. Warshel, Journal of Molecular Biology 224 (1992)7-14. [5] J. Åqvist, A. Warshel, Biochemistry, 28 (2002) 4680-4689. [6] M. Eigen, L. De Maeyer, Zeitschrift für Elektrochemie, Berichte der Bunsengesellschaft für physikalische Chemie 59 (1955) 986-993. [7] G. Oanca,A. Prah, Åqvist, J. Mavri, to be published

Understanding Non-Covalent Interactions via Mass-Selective Laser Spectroscopy in Jets

• Jose A. Fernandez (University of the Basque Country)

The study of molecular aggregation is decades old. The intermolecular forces that govern such process are small in module but big in importance, as they model the world around us. They also control key processes in the cell, such as docking or protein folding. For many years, there has been an intense activity in the field of spectroscopy in jets to characterize molecular aggregates of different nature and increasing size, to understand the above-mentioned process. The use of jets is required to create the necessary conditions for the molecules to aggregate. A cold environment ensures the survival of the species created during the cooling process at the exit of the valve’s nozzle. Then, several spectroscopic techniques have been developed to extract physical observables from the system, which are afterwards compared with high-level quantum-mechanical calculations, but the most popular ones are laser spectroscopy with mass-resolved detection and microwave spectroscopy. Both offer complementary data over the same systems. Using this methodology, many systems have been characterized. Here, we will review some of them and the lessons learned from their study, trying to create general rules of broad application.

H-Bonds in Photo- and Ion Pair Catalysis: NMR Spectroscopy as Mechanistic Tool

• Ruth M. Gschwind (University of Regensburg,)

H-bonds play a crucial role in many mechanistic aspects of catalytic reactions. However, in many rapidly expanding fields of catalysis and asymmetric catalysis, mechanistic studies as well as structural investigations on intermediates, intermolecular interactions or aggregate formation are scarce. In addition, to dissect the contribution of H-bonds in these mechanistic cycles is extremely challenging. In this talk I will present techniques and methods to get insights via NMR spectroscopy into various H-bond contributions in photocatalysis and ion pair catalysis and explain their impact on examples. Hydrogen bond assisted ion pair catalysis will be a main topic in this talk. In depths studies of the H-bonds between the catalyst and the substrate and their characterization will be addressed. Kinetic studies show that the intrinsic H-bond strengths is responsible for activation of the substrates, which is deviating from external measurements. Furthermore, the structure stabilization via a strong H-bond explains the broad substrate scope of the reaction. Beyond the binary catalyst substrate complexes also ternary intermediates are discussed including special relaxation optimized NMR pulse sequences enabling structure determination using the special properties of H-bonds. In addition, the SOFAST method can be used to detect megnetisation transfers via H-bonds even without 15N labelling. In photocatalysis it will be shown that H-bond analysis can be used to reveal so far hidden properties of well known H atom transfer reagents. There NMR spectroscopy can be even used to reveal indirect insights about the light part of a photocatalytic reaction. Last but not least first attempts to detect H-bond activation at oil water interfaces in photochemical reactions will be presented.

OH…NO! Investigating the interactions between TEMPO and large halogenated alcohols

• Elisabeth Sennert (Georg-August-Universität Göttingen)

Motivated by a large dynamical nuclear polarization effect observed for aminoxyl (NO) radicals combined with halogenated solvents [1] and the investigation of the microsolvation of TEMPO in the gas phase by Brás et. al. [2][3], the present work investigates the interactions between TEMPO and (halogenated) benzyl alcohol derivatives. The OH stretching vibration of the alcohol is used as a sensitive probe for the presence of a specific heterodimer (1:1) conformation by comparing slit jet FTIR-spectra with the results of harmonic DFT calculations. When TEMPO is combined with benzyl alcohol, two conformers with different ring stacking variants are observed. The preferred stacking can be controlled in subtle ways by introducing a halogen (F, Cl, Br, I) to the *ortho*- or *para*-position of the aromatic system of benzyl alcohol. Variation of the halogen size and substitution position uncovers the interplay between hydrogen bonding and dispersion forces. [4] [1] G. Liu, M. Levien, N. Karschin, G. Parigi, C. Luchinat, M. Bennati, *Nat. Chem.* 9 (**2017**) 676. [2] E. M. Brás, T. L. Fischer, M. A. Suhm, *Angew. Chem. Int. Ed.* 60 (**2021**) 19013. [3] E. M. Brás, C. Zimmermann, R. Fausto, M. A. Suhm, *Phys. Chem. Chem. Phys.* 26 (**2024**) 5822. [4] E. Sennert, G. Bistoni, M. A. Suhm, *J. Phys. Chem. A* 129 (**2025**) 1648.

Hydrogen bonding enables detection of Elusive conformers: interplay between supersonic jet relaxation and complex formation in 4-(4-methoxyphenyl)aniline

• PABLO PINACHO MORANTE (University of Valladolid)

Understanding conformational relaxation pathways is essential for characterizing flexible molecular systems with multiple low-lying rotamers in supersonic jets. To investigate in depth the relaxation pathways, the conformational dynamics of 4-(4-methoxyphenyl)aniline (4-MPA), a newly synthesized molecule, were studied in a supersonic expansion using high-resolution rotational spectroscopy in the 2-8 GHz range. Only the most stable of two potentially populated conformers of 4-MPA was detected in the spectrum, with the absence of the higher-energy form attributed to collisional conformational relaxation in the supersonic jet. Using quantum-chemical calculations, four plausible pathways for the conversion of the higher-energy conformer to the global minimum were identified. Among them, only one involves a feasible motion with a low barrier consistent with relaxation, while the other pathways present rather high energy barriers. Upon complexation with water, the formation of a hydrogen-bonded complex alters the potential energy surface, hindering the relaxation. The complexation through hydrogen bonding traps the higher-energy conformer in the first stages of the supersonic expansion, before the relaxation processes occur, enabling its detection. The interplay between relaxation and complexation highlights the role of microsolvation in modulating intramolecular dynamics and opens a possible route to detect otherwise non-detectable conformers

Effect of microhydration on the conformational preferences of a capped dipeptide in the gas phase

• Aloke Das

Studies on the structures and conformations of microsolvated peptides in the gas phase are scarce in the literature. This scarcity arises primarily from the difficulty of achieving sufficient cooling during jet expansion when laser desorption is used to vaporize peptides and generate microhydrated peptide clusters. In this work, we investigate the effect of microhydration on the secondary structure of a capped dipeptide, Boc-DPro-Gly-NHBn-OMe (Boc = tert-butyloxycarbonyl, Bn = benzyl), i.e., Pro–Gly (PG), in the presence of a single H₂O molecule using gas-phase laser spectroscopy combined with quantum chemical calculations. In the gas phase, the PG monomer adopts a C7–C7 conformation, whereas in the condensed phase, it assumes a β-turn structure.1 IR–UV hole-burning spectroscopy of PG⋅⋅⋅(H₂O)₁ confirms the observation of a single conformer in the experiment. Both experimental and theoretical IR spectra demonstrate that the H₂O molecule is selectively inserted into the relatively weak C7 hydrogen bond (γ-turn) formed between the Pro C=O and NHBn N–H groups, while the other C7 hydrogen bond (γ-turn) between the Gly N–H and Boc C=O groups remains unaffected. Consequently, the single H₂O molecule in the PG⋅⋅⋅(H₂O)₁ complex significantly distorts the peptide backbone without appreciable alteration of the overall secondary structural motif (γ–γ) of the isolated PG monomer.2 This study of the monohydrated peptide suggests that multiple water molecules may be required to switch the secondary structure of PG from the double γ-turn to the β-turn conformation that is favored in the condensed phase. Future work is in progress to study the conformations of PG with a larger number of water molecules. References 1. S. Kumar, K. Borish, S. Dey, J. Nagesh, and A. Das, Phys. Chem. Chem. Phys. 2022, 24, 18408. 2. S. Mandal, A. Kossov, P. Carcabal, and A. Das, J. Chem. Phys. 2024, 161, 214304.

Dynamics of microsolvated (bio)molecules in general-chemistry conditions

• Jochen Küpper (CFEL, DESY & UHH)

Observing molecules in action through the recording of “molecular movies”, i.e., their spatiotemporal evolution during chemical dynamics, with atomic spatial and temporal resolution promises to revolutionize our understanding of the molecular sciences and to provide a time-dependent basis of chemistry. However, most *real-world* chemistry occurs at or near room temperature, yet the ultrafast dynamics of corresponding elementary chemical processes at this energy scale are largely unexplored. We aim to change this [1]. Experimentally, we build upon our approaches to prepare highly controlled samples that enable advanced imaging methods of individual molecular species and directly in the molecular frame. We prepare highly-controlled molecular samples for advanced ultrafast imaging experiments. This includes the preparation of ensembles of individual molecular species, e.g., single microsolvation environments, single conformers, or even single quantum states. Furthermore, the generated very cold samples are ideally suited to fix the molecules in space in laser-alignment or mixed-field orientation approaches. I will discuss how we can utilize these highly controlled, ultracold samples to investigate "room-temperature" chemical dynamics. I will present first experimental results and discuss both the chemical information obtained as well as the challenges ahead for disentangling ultrafast elementary steps of general-chemistry in general. [1] M. S. Robinson and J. Küpper, Unraveling the ultrafast dynamics of thermal-energy chemical reactions, *Phys. Chem. Chem. Phys.* **26**, 1587 (2024).

Preference of hydrogen bond binding site in 1:1 chromone-methanol complexes in different electronic states

• FABIAN DIETRICH (Universidad de La Frontera)

The cluster formed by chromone and methanol serves as an excellent model for studying the various contributions to intermolecular interaction energy. The asymmetric ketone motif of chromone provides distinct hydrogen-bonding sites, enabling the differentiation between an "inside" and an "outside" isomer. We present the delicate balance between these two competing arrangements, investigated by combining IR/R2PI and UV/IR/UV spectroscopy in a molecular beam, supported by quantum-chemical calculations. Upon electronic excitation, chromone undergoes efficient intersystem crossing into the triplet manifold, allowing studies on aromatic molecule–solvent complexes to be extended to a cluster in a triplet state. We show that in both electronic states, the “outside” isomer dominates experimentally, in agreement with simulations performed at the DFT, SAPT0, and DLPNO-CCSD(T) levels. These findings raise the question of whether chromone derivatives could exhibit a change in binding site preference upon electronic excitation. To explore this, we screen a variety of chromone derivatives with functional groups at positions 2 and 6 of the chromone backbone. A detailed study using DFT simulations identifies three chromone derivatives that meet this criterion.

Conformer-sensitive nuclear dynamics of the ammonia dimer cation probed by femtosecond time-resolved Coulomb explosion

• Xiaoyu Mi (State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter)

Unlike in the liquid phase, the debate of whether and how hydrogen-bonded structures exist in a neutral ammonia dimer (NH3)2 in the gas phase has been ongoing for several decades. Here we distinguish the structures of neutral ammonia dimers with and without hydrogen bonds by photoionization, because the ions inherit initial structures from the neutral dimers and lead to significantly different Coulomb explosion channels in our pumpprobe experiment, i.e., the direct dissociation (NH3 + + NH3+) and indirect dissociation with proton migration (NH2+ + NH4+). With quantum chemical and molecular dynamics simulation, we showcase that these two different Coulomb explosion channels originate from the ammonia dimer cations with different structures. The dimer cations without hydrogen bonds correlate with the direct Coulomb explosion channel. In contrast, dimer cations with hydrogen bonds are likely to undergo ultrafast proton migration in ∼48 fs which has no potential barrier and correlate with the indirect dissociation channel in the Coulomb explosion. The 48 fs characteristic time is used to exclude the slower indirect dissociation initiating from non-H-bonded cations. Our work demonstrates a highly sensitive approach to probe weakly bonded and fluxional structures of gas phase molecular clusters by utilizing both channel- and time-resolutions of Coulomb explosion.

Efficient Quantum Chemical Methods for Non-covalent Interactions

• Stefan Grimme (University Bonn)

Non-covalent interactions (NCI) play a crucial role across many areas of science, and their theoretical description and quantification remain key topics in quantum chemistry. This talk focuses in particular on the treatment of long-range (London) and medium-range dispersion (correlation) effects within the framework of Kohn-Sham density functional theory (DFT). In recent years, it has become evident that these effects are not limited to traditional van der Waals systems but also significantly impact both weak and strong hydrogen bonds, as well as conventional thermochemistry. The presentation will begin with an overview of the current status of DFT in describing NCIs and accounting for dispersion effects. The importance of rigorous benchmarking against accurate, preferably CCSD(T)-level, reference data will be emphasized. General methodological recommendations for applying quantum chemical methods to NCIs will be discussed. Finally, a range of dispersion-corrected DFT results will be presented, including efficient composite, so-called 3c-approaches and our latest developments in the tight-binding framework, specifically the g-xTB method. It enables the accurate and routine treatment of very large systems containing up to a few thousand atoms.

Decoding Hydrogen-Bonded Clusters: Structural Insights from Rotational Spectroscopy

• Maria Eugenia Sanz (King's College London)

The investigation of hydrogen-bonded clusters in the gas phase, where the effect of the environment is removed, is crucial to understand the interplay of intra- and intermolecular forces and how modest changes influence structural outcomes. Rotational spectroscopy, with its direct dependence of mass distribution and its unparallel capability to differentiate between conformers and isomers, is an ideal technique to study such clusters. Specifically, microsolvated clusters where water is added to solutes in a stepwise fashion provide insight into how the transition from isolated to fully solvated molecule proceeds. In this talk we will discuss our findings on several clusters of water with molecules of atmospheric and biological interest, investigated by a combination of broadband rotational spectroscopy in supersonic jets and quantum chemical calculations. We will comment on the preferred structural choices and relevant non-covalent interactions, as well as on the performance of the various theoretical approaches.

MAKING HYDROGEN BONDS: REACTION KINETICS OF HETERODIMER FORMATION

• Alberto Macario Farto (Laboratoire de Physique des Lasers - Université Sorbonne Paris Nord)

The determination of the structures of hydrogen bonded complexes has been a major field of research in gas-phase chemistry. By contrast, the characterization of the process of complex formation has barely been addressed, despite its importance. Complex formation is a key step in homogeneous nucleation and particle formation, for example in cloud formation in planetary atmospheres and dust particle inception in circumstellar shells. Moreover, under the appropriate conditions, low temperatures and/or high degrees of supersaturation, the formation of the first complex, the dimer, is the-rate limiting step of nucleation, which can become a barrierless process. A few previous studies have been performed on the kinetics of formation of homodimers. These studies used the CRESU (reaction kinetics in uniform supersonic flow) technique mainly coupled with mass spectrometric detection and high-level quantum calculations and models. Here we use a completely new detection scheme, chirped-pulse FTMW spectroscopy, to study the kinetics of heterodimer formation. This innovative technique combines the ability to generate continuous cold uniform supersonic flows with the high selectivity and general applicability of MW spectroscopy, allowing the simultaneous following of both reactant and products. Moreover, the high sensitivity achieved allows us to employ pseudo-first-order conditions to obtain absolute rate constants. Quantum chemistry calculations have been performed in order to model and better understand the reaction paths of the complexation process. We report the measurements of rate constants for the formation of formic acid-CO2 heterodimer in a range of different pressures and temperatures. All of these characterize for the first time the kinetics of heterodimer formation.

UV Photochemistry of cis-Pinonic Acid and of its Complexes with O2 and H2O in Cryogenic Matrices

• Stephane COUSSAN (CNRS-Aix Marseille University)

The UV photochemistry of cis-Pinonic Acid has been studied in cryogenic matrices, as well as that of these complexes with water and oxygen.This molecule, which is of interest in the atmosphere, is one of the products of the decomposition chain of alpha- and beta-pinenes into a tri-acid: MBTCA. This decomposition occurs through the successive or simultaneous action of hydration, ozonolysis, and solar irradiation. Understanding the role of water (and oxygene) coupled with solar irradiation during this aging process is particularly important to address isomerization or fragmentation processes. I will present the latest results obtained on this system, for which not even the infrared (IR) spectrum has been published.

The site of protonation and proton delocalization in gaseous molecules elucidated by IR spectroscopy

• Jos Oomens (FELIX Laboratory, Radboud University, Nijmegen, The Netherlands)

In electrospray ionization mass spectrometry (ESI-MS), molecules are usually ionized by protonation. The site of protonation is not necessarily the site of highest proton affinity, especially in cases where site-specific proton affinities in gas and solution phase deviate significantly. We use infrared ion spectroscopy employing the FELIX free-electron laser to determine the protonation site in the gas phase of the mass spectrometer. In addition to ion spectroscopy, various groups have employed ion mobility to differentiate between different protomers. We will relay recent experiments combining ion spectroscopy and ion mobility (as implemented on our modified Bruker SolariX FTMS platform) that indicate that post-mobility scrambling of the proton may occur. For systems with two (nearly) equally nucleophilic sites in close proximity, the ionizing proton may become shared between the two nucleophiles. We discuss the spectroscopic signatures of such delocalized protons in infrared multiple-photon dissociation (IRMPD) spectra. Quantum-chemical modelling provides a basis for understanding these spectra, where static versus dynamic approaches reveal interesting insights into the vibrations involving the delocalized proton.

Ligand exchange in a supersonic expansion

• Beppo Hartwig (Georg-August-Universität Göttingen)

The smallest possible molecule that involves hydrogen atoms forming a van der Waals complex is dihydrogen itself. Complexes of dihydrogen with other binding partners are complicated by the fact that H$_2$ can exist in two different nuclear spin isomeric forms, one of which must retain its rotation in the complex (*ortho*-H$_2$, *I* = 1) whereas the other does not (*para*-H$_2$, *I* = 0). This results in different physical behaviour and can even impact their reactivity in the gas phase.[1] Hence, seemingly simple complexes are not necessarily easy to understand and it is commonly observed that *ortho*-H$_2$ complexes are more stable than *para*-H$_2$. Inspired by a rotational study on the OCS-H$_2$ complex[2], we investigated the formation of the *ortho*-H$_2$, *para*-H$_2$ and He complex with benzonitrile dependent on the H$_2$ content in the expansion using cavity-resonator rotational jet spectroscopy. Similar to OCS-H$_2$, an enhancement of the signal intensity of the *ortho*-complex with increasing H$_2$ content was observed, whereas an inverse behaviour was found for the *para* and He-complex. To understand this behaviour we kinetically modelled their intensity on the basis of an often proposed ligand exchange mechanism. These results may have implications for the *ortho/para*-H$_2$ ratio observed in the interstellar medium (ISM) for which polycyclic aromatic hydrocarbons (PAHs) have been found to be important.[3,4] Benzonitrile can serve as a small model system for PAHs and has itself been detected in the ISM.[5] [1] Yang, T. et al. Nat. Chem. 2019 11, 744–749. [2] Z. Yu et al. J. Chem. Phys. 2007 127, 054305. [3] E. Bron et al. A&A 2016 588, A27. [4] B. Fleming et al. Astrophys. J. 2010 725, 159–172. [5] B. A. McGuire et al. Science 2018 359, 202–205.

H-bond Driven Diastereomer Formation in Chiral Selector Ion Vibrational Spectroscopy of Protonated Amino Acids

• Francine Horn (Leipzig University / Fritz Haber Institute)

Molecular chirality plays a central role in how the building blocks of life, like amino acids, sugars, or nucleotides, interact. The stereochemistry and conformational flexibility of chiral molecules have a strong impact on their biological, biochemical, and pharmacological properties. A central analytical challenge is the generally applicable differentiation of enantiomers, as well as the fast and accurate determination of the enantiomeric excess of a chiral sample. Gas phase Chiral Selector Ion Vibrational Spectroscopy is a highly sensitive, selective, and fast tool for this purpose. Chiral protonated amino acids are transferred into the gas phase, where they interact with volatile chiral selector molecules in a gas-filled ion guide under the formation of diastereomeric complexes. These are then mass-selected, cryogenically cooled, messenger-tagged and an infrared photodissociation (IRPD) spectrum is measured. The spectra of the vibrationally cold diastereomers exhibit sufficiently different IR fingerprints, such that they can be spectrally distinguished and quantified. We study how to maximize the differences in the IRPD spectra of the diastereomers and gain insights into the chiral recognition process. For this purpose, a set of chiral selectors and chiral amino acids with different structural motifs and different number of stereocenters is investigated. We identify the intermolecular non-covalent interactions at work, with H-bonds being the most decisive for diastereomer formation.

Comparative Conformational Analysis of IAM and IAA and Related Compounds: Structural Insights into Their Biological Roles

• SOFIA MUNICIO SAN JOSE (Universidad de Valladolid)

This work presents a comparative rotational spectroscopic study of two biologically relevant indole derivatives: 3-indoleacetic acid (IAA) and indole-3-acetamide (IAM), analyzed using chirped-pulse Fourier transform microwave (LA-CP-FTMW) spectroscopy in the 6–14 GHz frequency range. Based on previous studies focused on the individual structural characterization of serotonin, an important biomolecule, our approach emphasizes a structural and conformational comparison of IAA and IAM with related systems originating from the same precursor. Although these compounds participate in different biosynthetic pathways—IAA and IAM in auxin biosynthesis, and serotonin in neurotransmission—they all share tryptophan as a common biosynthetic precursor. By analyzing their intramolecular interactions, particularly hydrogen bonding, we investigate how these forces influence the conformational landscapes of each molecule. Our results reveal significant differences in conformational preferences and intramolecular stabilization mechanisms, which may be closely related to their distinct biological functions. This comparative approach provides valuable structural insights into how small variations in functional groups can affect the molecular behavior of indole derivatives in biological systems.

H···H Dihydrogen Bonding in Alkylsilane–Fluoromethane Complexes Observed by Microwave Spectroscopy

• Weixing Li (Department of Chemistry, Fudan University)

Dihydrogen bonds (DHBs) are non-covalent interactions characterized by the X–Hᵟ⁺⋯ᵟ⁻H–Y motif, where X is an electronegative atom and Y an electropositive center (e.g. transition metals). In this work, we investigate DHB formation in clusters of multifluoromethanes (acting as proton donors) with trimethylsilane (TMS) and triethylsilane (TES) as hydrogen bond acceptors. Using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy, we unambiguously identified DHBs via rotational transitions. Notably, spectral splittings reveal quantum tunneling dynamics in these complexes, indicating coherent interconversion between degenerate configurations.

Conformational preferences of jet-cooled Gly-Gly-Gly and Ala-Ala-Ala peptides

• Vitali Zhaunerchyk (Department of Physics, University of Gothenburg, Sweden)

Proteins play an essential role in the functioning of living organisms. The function the protein performs is determined by its three-dimensional structure assumed upon folding. Hydrogen bonding is a key interaction in stabilizing protein structures. Spectroscopy of gas-phase peptides provides a bottom-up approach to studying protein secondary structures at the fundamental level. Among the different spectral regions, IR spectroscopy of jet-cooled molecules, when combined with quantum chemical calculations, is a particularly sensitive tool for conformational analysis of gas-phase peptides. A traditional method to measure IR spectra of gas-phase molecules is UV-IR double-resonance spectroscopy. The advantage of this method is that it is conformer-specific, but since only 3 proteinogenic amino acid residues can act as UV-chromophore, its application to peptides is rather limited. To overcome this drawback, we have introduced IRMPD-VUV spectroscopy which can be applied to neutral molecules of arbitrary structures, including chromophore-free peptides. In my presentation I will show the results from our recent FELIX (Free Electron Lasers for Infrared eXperiments) experimental campaign in which we investigated conformational preferences of jet-cooled Gly-Gly-Gly and Ala-Ala-Ala tripeptides. The IRMPD-VUV spectra have also been measured in the amide-A range, which was possible due to the recent extension of the FELIX range to $\approx$3700 cm$^{-1}$. The obtained spectra have been interpreted with the help of DFT calculations which suggest that, in contrast to the Gly-Gly and Ala-Ala dipeptides, both Gly-Gly-Gly and Ala-Ala-Ala assume only bent structures in the gas phase. I will also present the results of the hydrogen-bond analysis undertaken with the NCI method.

Benchmarking quantum methods with experimental data: the rotational spectroscopy of 3-fluorobenzylamine and its water complexes

• Sonia Melandri (University of Bologna)

The study of small clusters in the gas phase offers a unique opportunity to characterize both experimentally and theoretically models of the molecular interactions which occur in bulk water solutions. The conformational space of such systems is generally shaped by non-covalent interactions (NCIs) occurring within the molecule or with the surroundings. Moreover, they usually possess a high number of low-energy conformations undergoing large amplitude motions through shallow potential energy surfaces. For these reasons, the properties of such systems are difficult to predict and are challenging for theoretical calculations. High-resolution spectroscopy, in particular rotational spectroscopy, allows the deduction of several molecular properties such as structural parameters (e.g. the moment of inertia, electric dipole moments, centrifugal distortion and hyperfine coupling constants (i.e., nuclear quadrupole, spin-rotation, and spin-spin quadrupole) which can be very well determined, both numerically and also in terms of their physical description. The results of high-resolution spectroscopy studies can be directly compared to the outcomes of theoretical calculations as regards the energy order of the stable configurations and their geometries. Consequently, they offer valuable insights into the underlying driving forces governing interactions between water and solute. The rotational spectroscopy 3-fluorobenzylamine and its water complexes is presented. Very detailed structural information is obtained from the observation of the normal and isotopic species which, integrated with the results of quantum chemical calculations yielded valuable data for structural determination. In addition, these studies also consider the impact of the weak interactions and large amplitude motions and structural averaging correction.

The Twisting World of Chirality: A Molecular Perspective

• Yunjue Xu (Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2)

In 1848, French chemist Louis Pasteur made a groundbreaking discovery when he separated crystals of tartaric acid into two distinct mirror-image forms, marking the birth of molecular chirality. Since then, researchers have embraced the challenge of distinguishing left- and right-handed molecules, as their unique chiral non-covalent interactions can lead to vastly different biological and pharmaceutical effects. How can we tell them apart? In this talk, I will explore how modern spectroscopic techniques, in concert with theoretical modelling, enable us to probe chirality recognition, transfer, and amplification at the molecular level. I will illustrate this through three case studies: a gas-phase rotational spectroscopic analysis of quantum tunneling effects in a chiral dimer[1]; a vibrational circular dichroism (VCD) study of a flexible salen ligand revealing drastic solvent effects; and a combined VCD and electronic circular dichroism (ECD) investigation of an atomically-precise chiral metal cluster, showing how ligand conformation governs chiral spectroscopic response and drives chirality transfer and amplification. These examples highlight the central role of non-covalent interactions, advancing our fundamental understanding of chirality and its broad relevance.

Probing Noncovalent Interactions by Fast Magic-Angle Spinning NMR at 100 kHz and More

• Thomas Wiegand (RWTH Aachen University & Max Planck Institute for Chemical Energy Conversion)

Molecular-recognition events play a key role in biology and chemistry and are driven by noncovalent interactions, such as hydrogen bonding or dispersion interactions. Proton nuclear spins in NMR experiments serve as highly sensitive reporters for such weak chemical interactions. Only recently, the technically achievable magic-angle spinning (MAS) frequencies have become high enough to efficiently average out the dipole couplings in the dense proton dipolar network, which otherwise led to rather broad and unresolved 1H resonances in solids blurring the information on noncovalent interactions. I will discuss the benefit of proton-detected solid-state NMR experiments at MAS frequencies of 100 kHz and more and at high static magnetic-field strengths in chemical and biological applications emphasizing effects of weak interactions. The examples I will focus on range from hydrogen-π and cation-π interactions in calix[4]arene-based lanthanide complexes to hydrogen bonding in nucleic-acid binding to large ATP-fueled motor proteins. Of particular interest in this vein is to explore the temperature dependence of proton chemical-shift values, which can be diagnostic for hydrogen-bond formation.

Hydrogen-Bond-Mediated Conformational Locking and Quantum Tunnelling Effects in a Ferrocene Based Molecular Rotor

• Nuno Campos (CFisUC)

Despite being driven by quantum processes, most synthetic molecular machines exhibit classical kinetics, whereas operation by quantum tunnelling is largely elusive. In a recent investigation into the dynamics of metallocene molecular rotors, we found evidence of quantum tunnelling effects in the rotational spectrum of 1,1’-ferrocenedimethanol. Metallocenes are organometallic compounds with two nearly parallel rings surrounding a central metal atom. The simplest and best-known example of this group of sandwich compounds [1] is ferrocene, where two cyclopentadienyl rings freely rotate around a central Fe(II) atom, adopting staggered or eclipsed configurations. The rotational behaviour and associated energy barrier of these molecules depends on the substituents introduced to the rings. Bulky groups tend to increase the rotational barrier, often locking the molecule into specific configurations [2]. However, in some cases, such as phenyl substitution, unexpected effects like concerted ring rotation can occur [3]. In this contribution, we discuss a topology where a hydrogen bond between hydroxymethyl groups locks the system into a staggered or eclipsed configuration. We present experimental data from jet-cooled broadband rotational spectroscopy and discuss theoretical strategies used to interpret the observed tunnelling effects using complementary quantum chemistry calculations. References [1] Browne, W. R.; Pijper, D.; Pollard, M. M.; Feringa, B. L.; Synthetic Molecular Machines, based on noninter-locked molecules; from concepts to applications. Solvay Conference. Wiley, 2008. [2] Luke, W.D.; Streitweiser, A.; J. Am. Chem. Soc., 1981, 103, 3241–3243. [3] Castellani, M.P.; Wright, J.M.; Geib, S.J.; Rheingold, A.L.; Trogler, W.C.; Organometallics, 1986, 5, 1116–1122.

HYDROGEN BOND POTENTIALS FOR HCN/HNC COMPLEXES

• ELANGANNAN ARUNAN (Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India.)

SHASHANK SAHU, AMRESH KUMAR, ELANGANNAN ARUNAN, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India. Hydrogen bonding controls the properties of water and DNA, molecules of life, and has received enormous attention in the last century. Yet, it does not have a universally accepted potential due to its dependence on various physical and chemical forces, such as electrostatics, polarization, dispersion, exchange repulsion, and charge-transfer covalency. Recently, Shahi et al.1 and Hays et al.2 observed binding energy and collisional excitation properties for CN/HCN complexes and noted that HNC has stronger interaction than HCN, though both have nearly identical dipole moment. Theoretical estimates on charges indicated that the H in HNC has higher charge than that in HCN. This suggested that the HCN/HNC interaction could be charge-dipole and the potential could show r􀀀2 dependence. We studied the potential energy surface of HCN/HNC complexes with different HB acceptors such as HF, HCN, HNC, C2H4, C2H2 and Ar. The first three acceptors have strong dipole moments, the next two have a quadrupole moment, and Ar is a spherical electron cloud. Depending on the acceptor, the interactions could vary as charge-dipole, charge-quadrupole and charge-induced dipole. The attractive part of the potential would then be expected to vary. This talk will summarize our results, which confirm that hydrogen bonding cannot be characterized as a specific physical force. References: 1. Shahi, A.; and Arunan, E., Physical Chemistry Chemical Physics 16(42), 22935–22952 (2014). 2. Hays, B.M.; Gupta, D.; Guillaume, T.; Khedaoui, O. Abdelkader; Cooke, I.R.; Thibault, F.; Lique, F.; Sims, I.R. Nat Chem 14(7), 811–815 (2022).

Hydrogen bonding motifs in hydroxy- and carboxy-functionalized ionic liquids

• Ralf Ludwig (Universität Rostock)

The unique properties of ionic liquids (ILs) result from the tunable mélange of Coulomb interactions, hydrogen bonding, and dispersion interactions among the constituent ions. In hydroxy-functionalized ILs, local and directional hydrogen bonds (H-bonds) lead to the anticipated formation of ion pairs but also to the elusive formation of cationic clusters. Here, we report how hydrogen-bonding motifs in the bulk liquid and gas phase of hydroxy-functionalized ILs shed light on the general nature of hydrogen bonding.[1] Infrared spectroscopy, nuclear magnetic resonance, neutron diffraction, and molecular dynamics simulations provide information about the structure, strength, and dynamics of cationic clusters in the bulk liquid ILs. Cryogenic ion vibrational predissociation (CIVP) spectroscopy along with density functional theory calculations has established a clear picture about the specific contacts within isolated H-bonded cationic clusters formed in the gas phase. For carboxy-functionalized ILs we provide experimental evidence for doubly H-bonded cationic dimers in the solid, liquid, and gaseous phases.[2-4] This information from experiment, simulation, and theory provides a fundamental understanding of hydrogen bonding between ions of like charge. References 1. Strate, A.; Paschek, D.; Ludwig, R. Ann. Rev. Phys. Chem. 2025, 76, 589–614. 2. Khudozhitkov, A. E.; Hunger, L.; Al Sheakh, L.; Stepanov, A.G.; Kolokolov, D.I.; Ludwig, R. Phys. Chem. Chem. Phys. 2025, 27, 5949–5955. 3. Hunger, L.; Al Sheakh, L.; Fritsch, S.; Villinger, A.; Ludwig, R.; Harville, P.; Moss, O.; Lachowicz, A.; Johnson, M. A. J. Phys. Chem. B 2024, 128, 5463– 5471. 4. Harville, P. A.; Moss, O. C.; Hassan, Y.; Hunger, L.; Ludwig, R.; McCoy, A. B.; Johnson, M. A. J. Phys. Chem. A 2024, 128, 10159–10166.

Bridging Non-Covalent Interactions in Diverse Cyclic Systems: Insights from Rotational Spectroscopy and Quantum Chemistry Calculations

• MAIDER PARRA SANTAMARIA (Group of Spectroscopy, Spectrometry, and Materials; University of the Basque Country (UPV/EHU))

The conformational behaviour of biologically relevant cyclic molecules raises fundamental questions from a physicochemical perspective, as their structural and energetic features are closely linked to physicochemical properties and biological function. These systems are often classified by ring size into small (3–7 members), medium (8–11), and large (>12). However, as molecular flexibility increases, identifying energetic minima becomes challenging due to the interplay of weak and competing intramolecular interactions.[1] Despite their relevance, conformational studies—particularly on larger rings and their hydrated complexes[2]—remain scarce. This work summarises key findings from recent years on non-covalent interactions that govern structural preferences, the role of functional group modifications, and the effects of microsolvation on a series of cyclic systems of increasing size, including their water complexes. These studies employ high-resolution rotational spectroscopy combined with quantum chemical calculations, showing this approach is a powerful tool for elucidating the subtle but crucial structural and interaction features that define the behaviour of these systems. [1] V. W. Tsoi, E. Burevschi, S. Saxena and M. E. Sanz, J. Phys. Chem. A, 2022, 126, 6185–6193. [2] C. Pérez, J. C. Lopez, S. Blanco and M. Schnell, J. Phys. Chem. Lett., 2016, 7, 4053–4058

Mechanical tuneability via water-induced H-bond change in sustainable soft materials

• Kai Zhang (University of Göttingen)

Water is present in many soft matters in our daily lives, and affects their many properties, among others the morphology and mechanical properties. Either in hydrogel or in membranes/films of hydrophilic compounds, water will strongly affect these aspects of materials via alternative absorption and desorption steps. Using native compounds for the construction of soft materials, such as cellulose or nanocellulose derivatives, their mechanical properties are found to be strongly affected by water absorption and desorption, which in turn is mediated by H-bond change. By using dynamic mechanical analysis in combination with dynamic vapor sorption and spectroscopic methods, correlations between mechanical properties and water sorption/desorption as well as induced H-bond changes for certain materials are intended to be established.

BENZOIC ACID MICROSOLVATION IN GAS PHASE BY ROTATIONAL SPECTROSCOPY: STRUCTURE AND COOPERATIVE EFFECTS

• ROGER CASTILLO DELGADO (Universidad de Valladolid)

Gas-phase hydration of carboxylic acids, such as benzoic acid (C₆H₅COOH, BzAc), is crucial for a better understanding of biochemical processes in condensed phases. Despite its interest only a microwave rotational study of its monohydrated complex has been reported so far.$^i$ Herein, we present our investigation of higher-order benzoic acid hydrates, generated in a supersonic jet expansion and characterized using broadband chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW). We have identified several new benzoic acid hydrates in the gas phase: BzAc-W₂₋₅ and BzAc₂-W. Additionally, we detected spectra for all ¹³C monosubstituted isotopologues of BzAc, and the H₂¹⁸O monosubstituted isotopologues for BzAc-W₂ and BzAc₂-W, which enabled the determination of their Kraitchman substitution structure. Furthermore, we extended the measurements for the reported species across the 2–8 GHz frequency range. The BzAc-W₃ spectrum reveals a tunneling splitting between two equivalent non-planar forms, analyzed using a Coriolis-coupled two-states Hamiltonian to determine the vibrational energy spacing. Our experimental results have been complemented with theoretical calculations, providing a deeper understanding of the forces stabilizing these complexes and exploring the relationship between hydration number, structure, and cooperative effects. $^i$ E. G. Schnitzler and W. Jäger, The benzoic acid–water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations, Phys. Chem. Chem. Phys., 16(6), 2305-2314, 2014. DOI: 10.1039/C3CP54486A.

Invisible Forces, Precise Spectra: A Journey Through Noncovalent Interactions via Gas-Phase Rotational Spectroscopy

• Camilla Calabrese

Hydrogen bonding is one of the most fundamental and widespread noncovalent interactions in chemistry and biology, governing the structure, dynamics, and function of countless molecular systems. Despite its apparent simplicity, hydrogen bonding displays remarkable diversity in geometry, strength, and electronic character, especially when studied in isolation from environmental effects. Rotational spectroscopy in the gas phase provides an exceptionally precise and unambiguous way to characterize hydrogen bonds. By isolating molecular complexes in supersonic expansions, it becomes possible to access their intrinsic structural and energetic features, free from solvation or packing forces. The analysis of rotational constants, nuclear quadrupole interactions, and dipole moment components offers detailed insight into bond directionality, donor–acceptor distances, and subtle electronic rearrangements induced by hydrogen bonding. In this contribution, several gas-phase studies of hydrogen-bonded complexes involving small organic molecules and relevant ligands such as water are presented. The role of secondary interactions and cooperative effects will also be discussed, particularly in systems where multiple H-bonds coexist or compete with other weak forces. Special attention is given to fluorinated aromatic acceptors, where hydrogen bonding competes with lone pair···π-hole interactions, offering a rich scenario for structural analysis. These experimental results, supported by high-level quantum-chemical calculations, provide benchmark data for understanding hydrogen bond geometries and for validating theoretical models. This journey through “invisible forces” demonstrates the powerful synergy between microwave spectroscopy and theoretical chemistry in elucidating fundamental aspects of molecular aggregation.

Peculiarities of Intramolecular Hydrogen Bonds in Porphyrin Isomers

• Jacek Waluk (Institute of Physical Chemistry, Polish Academy of Sciences)

Owing to (a) differences in symmetry and geometrical parameters of the inner macrocycle cavity and (b) possibility of existence of various tautomeric forms, porphyrin (porphine) and its isomers: porphycene, corrphycene, hemiporphycene, and isoporphycene are very good models for understanding the relationship between geometry and intramolecular H-bond strength. I will present the results of experimental and theoretical studies that show the importance of the N–N distance and the NHN angle for proper characterization of intramolecular H-bonds. In contrast to the intermolecular H-bonds, where these parameters adjust to optimal values, the geometry of the intramolecular bonds is imposed by the topology of the inner macrocycle cavity. This may lead to such unusual phenomena as short H-bonds being weaker than longer ones. Moreover, tautomers that exhibit the strongest H-bonds may not correspond to the lowest energy structures, because of unfavorable effects related to electron density redistribution upon tautomeric conversion. Finally, the apparent strength of the intramolecular H-bond may either increase of decrease with excitation of certain vibrational modes.

IR Spectroscopy of Phosphoric Acid Clusters in Helium Nanodroplets

• América Torres Boy (Fritz-Haber Institut)

Phosphates are ubiquitous in nature and play a crucial role in biochemical processes such as protein synthesis, metabolism, and energy production. One unique property of phosphoric acid is that it has the highest intrinsic proton conductivity of any known substance and is used in low-temperature batteries as well as in phosphoric acid fuel cells (PAFCs). The detailed mechanism of the proton transport is, however, not yet fully understood. In this study, we examine mass-selected ionic clusters containing phosphoric acid in the gas phase using infrared action spectroscopy in helium nanodroplets. Using this technique, spectra can be obtained at a cryogenic temperature of 0.37 K, reducing spectral congestion and yielding well-resolved vibrational bands. Studying hydrogen-bonded systems at these temperatures allows for unique insights into their fundamental properties, enhancing our understanding of phosphoric acid chemistry and its interactions across various environments. Our investigation reveals molecular structures that can serve as calibration points for quantum chemistry calculations. The elucidation of experimental vibrational bands, the hydrogen-bond interactions between the two moieties, as well as the spectroscopic details, will be discussed.

Short hydrogen bonds in the solid state: new insights from computational, infrared, inelastic neutron scattering and nuclear quadruple resonance perspective

• Jernej Stare (National Institute of Chemistry, Ljubljana, Slovenia)

The extremely short hydrogen bonding featuring donor…acceptor separation of ~2.4 Å is still relatively poorly understood and poses a challenge for advanced experimental and theoretical treatments. Among major issues is the highly delocalized H-bonded proton, with complex dynamics driven by strong nuclear quantum effects. These systems, when paired with experimental data, provide a valuable benchmark for computational methods. Herein, we present different examples of systems containing very short H-bonds, including the picolinic acid N-oxide family, quinolinic acid, lithium hydrogenphthalate dihydrate and nitranilic acid hexahydrate. All of these systems are present in the crystalline solid state with extensive structural data available (X-ray and neutron diffraction), allowing for the construction of reliable structural models subject to periodic DFT calculations. For each of the presented systems we characterized the H-bonding by slightly different combination of experimental and computational methodologies. The latter not only proved to be of a great value for the interpretation of experimental observables (e. g., assignment of vibrational spectra), but can also provide substantial improvements in structural characterization of H-bonding, in particular determining the precise location of the proton. Among the employed experimental techniques, nuclear quadrupole resonance proved to represent a very sensitive and reliable probe for H-bond geometry. On the other side, inelastic neutron scattering vibrational spectroscopy can circumvent many drawbacks of optical spectroscopy due to simplicity of interactions between neutrons and atomic nuclei. In conjunction with advanced computational routines, both techniques provide accurate and reliable tools for detailed characterization of short H-bonds, striking a good balance between precision and computational efficiency.

Is water an ultraviscous liquid above 125 K? Experiments on Droplet Coalescence

• Thomas Loerting (University of Innsbruck)

Glassy water has been made for the first time in the laboratory in 1935. Its glass transition temperature was reported for the first time in the early 1980s. Right after the first report of the increase in heat capacity near 136 K in calorimetry experiments at standard heating rates (20 K min-1) the discussion about the nature of the glass transition has started, and this discussion is still going on. The major question is whether or not the glass transition thermodynamically connects the glass to the liquid. Arguments can be found in literature both in favor of only orientational motions being unlocked at the glass transition temperature and in favor of translational motions being unlocked at the glass transition. The latter means that the glass transition is coupled to an alpha-relaxation and viscosity. Yet, direct measurement of viscosity are missing up to the current day. Here I present our recent work on hyperquenched glassy water, which is composed of thousands of micron-sized droplets. Based on SAXS/WAXS measurements at PETRAIII and the EuXFEL and based on direct observation by cryo-microscopy we show that water droplets actually start to coalesce near the glass transition temperature, i.e., capillary forces typical of viscous liquid awaken near the glass transition temperature and cause the droplet interfaces to disappear. This represent the first demonstration of true viscous nature of glassy water in the glass transition temperature range

How water changes the photophysics of small model chromophores

• Sebastian Trippel (FS-CFEL-CMI (CFEL-CMI Controlled Molecule Imaging))

The properties of atoms and molecules strongly depend on their environment, with hydrogen bonds, in particular, playing an important role in chemistry and biochemistry. It is, therefore, of great interest to bridge the gap between single molecules and molecules in solvation. In our group, we focus on experimental studies of small, model-chromophore dynamics including one-to-one clusters of a (bio)molecule with a single water molecule attached in the gas phase [1], as well as liquid-phase studies [2]. We investigated indole and pyrrole, both relevant model systems for the photophysics of tryptophan, the most strongly near-UV absorbing amino acid. Here, we present results on the photo-induced dynamics of indole-water, pyrrole-water, and water-water dimer clusters, probed by NIR and x-ray laser sources in our lab and at large-scale facilities such as LCLS and the European XFEL. Furthermore, mid-infrared pump IR-probe experiments on indole-water mimicking thermal-energy chemistry will be presented. Recent progress on our newly developed transportable sample injectors and endstations (COMO and eCOMO) for experiments both at large-scale facilities and in-house will also be outlined, including the use of time- and position-sensitive detection schemes provided by Timepix3 cameras [3,4]. [1] M. Johny, C. A. Schouder, A. Al-Refaie, L. He, J. Wiese, H. Stapelfeldt, S. Trippel, J. Küpper, Phys. Chem. Chem. Phys. 26, 13118 (2024), arXiv:2010.00453 [physics] [2] L. He, S. Malerz, F. Trinter, S. Trippel, L. Tomaník, M. Belina, P. Slavíček, B. Winter, and J. Küpper, J. Phys. Chem. Lett. 14, 10499, (2023), arXiv:2205.08217 [physics.chem-ph] [3] W. Jin, H. Bromberger, L. He, M. Johny, I. S. Vinklárek, K. Długołęcki, A. Samartsev, F. Calegari, S. Trippel, and J. Küpper, Rev. Sci. Instrum. 96, 023305 (2025), arXiv:2406.16491 [physics.chem-ph] [4] L. He, et int (25 authors), M. Meyer, S. Trippel, and J. Küpper, Rev. Sci. Instrum. 95, 113301 (2024), arXiv:2405.06344 [physics.chem-ph]

Microwave Spectroscopic Characterization of Furan–Formic Acid Clusters: Large-Amplitude Motions and Noncovalent Interaction Analysis

• Jingling HONG

Proton transfer (PT) reactions are essential in many chemical and biological systems due to their critical roles in energy conversion processes. Uncovering the mechanisms of PT is vital for controlling tunneling rates and advancing tunneling applications. The formic acid dimer undergoes double proton transfer; as a result, it serves as a well-established prototype for studying tunneling mechanisms. Herein, we measured the rotational spectrum of the formic acid-furan system and identified the Furan$-$FA$_{n}$ clusters with $n=2-4$. The results reveal that in the Furan$-$FA$_{2}$ cluster, the tunneling dynamics become more intricate. Specifically, two large-amplitude motions are involved: the rotation of the furan molecule coupled with the double proton transfer within the formic acid dimer. For the larger clusters, Furan$-$FA$_{3}$ and Furan$-$FA$_{4}$, no distinct splitting patterns are observed under the resolution of the broadband microwave spectrum (approximately 15 kHz). However, the two structures differ significantly, exhibiting notable changes in the noncovalent interactions both among the formic acid molecules and between formic acid and furan. Therefore, we performed many-body energy decomposition (MBE) and noncovalent interaction (NCI) analyses to investigate the competition among different types of noncovalent interactions present in these species.

H-bonding in acid-glycol switchable emulsifiers enlightened with sum-frequency generation spectroscopy

• Ellen Backus (University of Vienna)

Switchable emulsifiers are commonly studied at the macroscopic scale without significant insight into the molecular properties and processes underpinning their behaviour. Our work investigate a switchable emulsifier comprising poly(methacrylic acid) (PMAA) and poly(ethylene glycol) methacrylate (PEGMA). At alkaline pH, the polymer produces stable emulsions with well dispersed droplets, whereas at acidic pH the droplets aggregate. Here, we use sum frequency generation (SFG) spectroscopy to develop a molecular level understanding of the surface of the emulsifier coated droplets. With SFG spectroscopy we basically obtain the vibrational spectrum of an interface. In detail, we monitor the C=O vibration of PMAA as a function of dilution with PEGMA. From the observed changes, it can be concluded that at pH 2.5, both MAA-EG H-bonds and MAA = MAA cyclic dimers contribute to droplet aggregation in the emulsion. In contrast, from SFG spectra in the C-H/O-H stretch region, it can be concluded that at pH 11.5 deprotonation and solvation of the PMAA component lead to a polymer restructuring. As a result, the PMAA chains form a negative shell around the droplets, electrostatically stabilising the emulsion in alkaline conditions. Therefore, we conclude that the behaviour of the polymer is not only due to a simple protonation/deprotonation of the MAA units, but it is a rather complex process involving polymer restructuring, and at least two types of inter-polymer interactions. Our findings not only provide a proof for the so far hypothesised molecular explanation for the behaviour of the PMAA/PEGMA BCS switchable emulsifier, but can also serve as guidance for the design of similar responsive systems with better efficiency in applications ranging from smart materials to drug-delivery.

Distinguishing cavity and non-cavity solvation structures of the hydrated electron

• Benjamin Fingerhut (Ludwig-Maximilians-Universität München)

Solvated electrons in aqueous solution are a prototypical low dimensional quantum system in interaction with a fluctuating many-body environment. Despite substantial theoretical and experimental effort, conflicting views prevail regarding the hydration structure of electrons in water, where cavity and non-cavity solvation structures have been suggested. We present first principles molecular dynamics simulations of the electron localization dynamics in liquid water, employing hybrid-meta-GGA and hybrid-GGA density functionals that both provide an excellent description of the liquid water structure. Nevertheless, characteristic differences occur regarding the localization dynamics and solvation structure of excess electrons. We identify perturbations of the local hydrogen bond structure of water due to the interaction with the excess charge that give rise to specific signatures in transient radial distribution functions. Respective signatures in simulated scattering patterns are compared to preliminary data obtained in a liquid phase UED early science campaign at SLAC. The results shine light on the coupling mechanism of the aqueous electron with its environment and provide microscopic insight into the dynamics of polaron formation in disordered condensed matter systems.

Synergy Effect of Hydrogen Bonding and Steric Interactions for Reactivity and Selectivity of Organic, Inorganic and Organometallic Compounds

• Alexander Antonov (University of Regensburg)

Non-covalent interactions are a highly promising tool for the development of transition-metal-free chemospecific synthetic transformations. Here we demonstrated the implementation of synergy between hydrogen bonding and steric interactions as a simple, precise and flexible synthetic toolbox, allowing the controlled transformation of organic, inorganic and organometallic species into hard-to-reach products. We show the transformation of *ortho*-dimethylaminoaryloximes and imines into nitriles and various nitrogen heterocycles under mild conditions. This diverse reactivity is activated via hydrogen bonding and facilitated via the buttressing effect of the substituents next to the NMe2 group. All transformations require only simple and easily available acids and solvents, which generally provide precise control over the direction of the reaction. We introduce the first example of H-bond-assisted chalcogen exchange between arsine oxides and phosphine selenides under mild conditions, providing a powerful approach for the synthesis of arsine selenides. This newly discovered reaction is applicable to various arsine oxides and phosphine selenides, although the use of reagents with bulky substituents significantly hinders its efficiency. We utilize the non-covalent Li···H interaction as a tool for the second lithiation of various lithionaphthalenes. A series of previously inaccessible 4,5-disubstituted derivatives of 1,8-bis(dimethylamino)naphthalene has been prepared in a good to excellent yield.