Speaker
Description
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.
| Keywords | Hydrogen bond: weak; State of system: gas |
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| This abstract is submitted for.... | HBond 2025 conference |
