Photochemistry of micro-solvated benzonitrile complexes with water/ammonia bordering on the formation of icy grains

Abstract

Polycyclic aromatic hydrocarbons (PAHs) act as condensation nuclei for water/ammonia molecules forming icy water clusters. In the context of small aromatic rings, we have theoretically examined complexes of benzonitrile with water/ammonia molecules in order to investigate the photochemical kinetics of the icy grain formation. First, we obtain the structure of energetically stable complexes containing different H-bonding patterns and analyze their effects in the infrared (IR) and vacuum ultraviolet (VUV) spectra, before and after ionization. Second, we evaluate the H-bonding reorientation kinetics of the complexes at low temperatures, which indicates that the process involving water occurs faster than the corresponding process involving ammonia. Finally, the thermal rate constants indicate which configurations are the most likely to be ionized under vacuum ultraviolet (VUV) irradiation. Our results are consistent with what has experimentally been observed for related complexes, and are useful as a starting point to investigate the process of the icy grains formation. This study provides insight into possible subsequent photochemical reactions occurring in the interstellar medium (ISM) or in jet-cooled molecular complexes.