ИСТИНА

Ethanol (C2H5OH) is one of the most important organic molecules widely occurring in different environments on Earth and in the interstellar medium (ISM). It is believed that this molecule can play a very important role in the formation of interstellar complex organic molecules (iCOMs). In particular, the radiation-induced chemistry of C2H5OH in the ISM may lead to a variety of ethers and esters, as well as other iCOMs essential for the origin of life. Thus, the mechanism of ethanol transformation induced by high-energy radiation in icy media of various compositions is of great interest to prebiotic astrochemistry. Despite the number of studies, there are significant gaps in understanding molecular mechanisms of the ethanol transformations induced by radiation of different energy in cold rigid media remain. In particular, the contribution of the primary and secondary processes to the radiation chemistry of ethanol molecules in various media is not fully clear. These issues may be partially resolved using matrix isolation studies with the detection of paramagnetic species by electron paramagnetic resonance (EPR) spectroscopy. In the present work, we focused on the investigation of the radiation-induced transformations of ethanol molecules isolated in solid noble gas matrices (Ar and Xe) at cryogenic temperatures. Matrix samples were obtained by deposition of gaseous mixtures (CH3CH2OH/Ng and CH3CD2OH/Ng 1/1000; Ng = Ar or Xe) onto a sapphire rod mounted in a closed-cycle helium cryostat. Complementary experiments were carried out with the addition of Freon-11 (CFCl3) as an electron scavenger. The deposited matrices were irradiated with X-rays (effective energy ca. 20 keV) at 7 K. The EPR spectra were recorded using an X-band (9.4 GHz) spectrometer (SPIN, Russia) with a 100 kHz high-frequency magnetic field modulation and appropriate microwave power (from 10 to 300 μW). The results of the matrix isolation experiments demonstrate that the radiolysis of ethanol molecules isolated in solid argon mainly leads to the C−C bond cleavage leading to the formation of CH3• (methyl) and HCO• (formyl) radicals. At the same time, the dominant product of ethanol radiolysis in a Xe matrix is α-hydroxyethyl radical (CH3•CHOH). 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 CH3•CHX species, and exhibit free rotation in solid xenon only at ca. 65 K. The experiments with the selectively deuterated ethanol (CH3CD2OH) provide solid indirect evidence for the primary formation of the ethoxy radical (CH3CD2O•) as a result of O−H bond cleavage, which converts to the α-hydroxyethyl radical due to isomerization occurring at 7 K. The observed effect of a matrix (Ar and Xe) on the occurring radiation-induced transformations was explained based on the significant role of “hot” ionic fragmentation and inefficient energy dissipation to medium in the case of Ar. The implications of the obtained results are discussed.