Direct evidence for a radiation-induced synthesis of acetonitrile and isoacetonitrile from a 1 : 1 CH4⋯HCN complex at cryogenic temperatures: is it a missing link between inorganic and prebiotic astrochemistry?

Abstract

Nitriles are important constituents of extraterrestrial media. Nitriles are supposed to play a crucial role in prebiotic chemistry occurring in the interstellar medium. In this work, we have investigated the low-temperature radiation-induced transformations of a 1 : 1 CH₄⋯HCN complex as a plausible precursor of the simplest nitriles using the matrix isolation approach with FTIR spectroscopic detection. The parent complexes isolated in a noble gas (Ng) matrix were obtained by deposition of the CH₄/HCN/Ng gaseous mixture and characterized by comparison of experimental complexation-induced shifts of the HCN fundamentals with the results of the ab initio calculations. It was found that the X-ray irradiation of low-temperature matrices containing the isolated 1 : 1 CH₄⋯HCN complex resulted in the formation of acetonitrile (CH₃CN) and isoacetonitrile (CH₃NC) and it appears to be the first experimental evidence for the formation of C₂ nitriles (acetonitrile and isoacetonitrile) from such a “building block”. Additionally, a 1 : 1 CH₄⋯HNC complex was tentatively assigned to the irradiated Ar and Kr matrices. It is demonstrated that the matrix has a strong effect on the CH₃CN/CH₃NC yield ratio, which dramatically increases in the row Ar < Kr < Xe. Also, the efficiency of the radiation-induced formation of the CH₄⋯HNC complex was shown to decrease from Ar to Kr. It is believed that the proposed pathway for acetonitrile formation may be a significant step in the radiation-induced evolution leading to complex organic molecules and biomolecules under astrochemical conditions. Furthermore, the obtained results provide a prominent example of the impact of very weak intermolecular interactions on the radiation-induced transformations in cold media.