An EPR study on the radiolysis of isolated ethanol molecules in solid argon and xenon: matrix control of radiation-induced generation of radicals in cryogenic media

This paper addresses the basic question of the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures with possible implications for astrochemical issues. The radicals produced by X-ray irradiation of isolated ethanol molecules (C2H5OH and CH3CD2OH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH3) and formyl (HCO) radicals resulting from the C-C bond cleavage were mainly produced in the case of solid argon, which was attributed to the significant role of hot ionic fragmentation and inefficient energy dissipation to medium. In contrast, irradiation in xenon results in the predominant formation of alpha-hydroxyethyl radicals (CH3CHOH or CH3CDOH(D) in the cases of C2H5OH and CH3CD2OH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CH3CD2O) radicals due to O-H bond cleavage, which convert to the alpha-hydroxyethyl radicals due to isomerization occurring at 7 K. The alpha-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH3CHX species, and exhibit free rotation in solid xenon only at ca. 65 K.

Automated summary

This paper investigates the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures, with implications for astrochemical issues. The production of different radicals resulting from X-ray irradiation of isolated ethanol molecules in solid argon and xenon matrices at low temperatures was characterized using EPR spectroscopy. The results showed that the type of matrix influences the formation of radicals, with solid argon favoring the production of methyl and formyl radicals, while xenon promotes the formation of alpha-hydroxyethyl radicals.