Zasimov Pavel V, Sanochkina Elizaveta V, Tyurin Daniil A, Feldman Vladimir I
Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia.
Phys Chem Chem Phys. 2023 Feb 8;25(6):4624-4634. doi: 10.1039/d2cp05356j.
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 (CHOH and CHCDOH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH˙) 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 α-hydroxyethyl radicals (CH˙CHOH or CH˙CDOH(D) in the cases of CHOH and CHCDOH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CHCDO˙) radicals due to O-H bond cleavage, which convert to the α-hydroxyethyl radicals due to isomerization occurring at 7 K. The α-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH˙CHX species, and exhibit free rotation in solid xenon only at 65 K.
本文探讨了化学惰性环境对低温下冰介质中孤立有机分子辐射诱导转变的影响这一基本问题，这可能对天体化学问题具有启示意义。通过电子顺磁共振（EPR）光谱对7K下固体氩和氙基质中孤立乙醇分子（CHOH和CHCDOH）经X射线辐照产生的自由基进行了表征。结果表明，在固体氩的情况下，主要通过C-C键断裂产生甲基（CH˙）和甲酰基（HCO˙）自由基，这归因于“热”离子碎片化的重要作用以及向介质的能量耗散效率低下。相比之下，在氙气中辐照会分别在CHOH和CHCDOH的情况下主要形成α-羟乙基自由基（分别为CH˙CHOH或CH˙CDOH(D)）。值得注意的是，对选择性氘代乙醇的实验提供了有力的间接证据，证明由于O-H键断裂会初步形成乙氧基（CHCDO˙）自由基，该自由基在7K时由于异构化而转化为α-羟乙基自由基。α-羟乙基自由基在低温下具有特定的刚性构象，甲基不旋转，这对中性CH˙CHX物种来说是一种不寻常的效应，并且仅在65K时在固体氙中表现出自由旋转。
