ИСТИНА

Acetaldehyde is one of the key small organic molecules involved in astrochemical and atmospheric processes occurring under the action of ionizing and UV radiation. While the UV photochemistry of acetaldehyde is well studied, little is known about the mechanism of processes induced by high-energy radiation. In this study, we performed the first systematic model study on the mechanism of chemical transformations of the CH3CHO molecules resulting from X-ray irradiation under the conditions of matrix isolation in different solid noble gases (Ne, Ar, Kr, and Xe) at 5 K. Matrix samples were obtained by deposition of gaseous mixtures (CH3CHO/Ng or CD3CDO/Ng 1/1000; Ng = Ne, Ar, Kr, or Xe) onto a cold KBr substrate mounted in a closed-cycle helium cryostat. The deposition temperatures were typically about 8, 18, 25 and 33 K for Ne, Ar, Kr and Xe matrices, respectively. The deposited matrices were irradiated with X-rays (45 kVp, anode current 80 mA, effective energy ca. 20 keV) to various absorbed doses (up to 111 kGy) at 5 K. Photolysis of the irradiated samples was performed using ARPL-STAR-3W LEDs (λD = 620 nm and λD = 520 nm). The radiation-induced species were characterized by FTIR spectroscopy. The results of the present study revealed that the principal radiolysis products were CO, CH4, H2CCO, H2CCO–H2, C2H2···H2O, CH2CHOH, CH3CO•, CH3•, HCCO•, and CCO. It was found that the dominant pathway of acetaldehyde degradation involves C–C bond cleavage leading to the formation of carbon monoxide and methane. The second important channel is dehydrogenation resulting in the formation of ketene, a potentially highly reactive species. The significant effect of the matrix media on the decomposition efficiency and distribution of the reaction channels was observed. Based on these observations, it was suggested that the formation of the methyl radical as well as vinyl alcohol and the C2H2···H2O complex presumably included a significant contribution of ionic pathways. The decomposition of CH3CO• radicals under photolysis with visible light leading to the CH3•–CO radical-molecule pair was found in all matrices, while the recovery of CH3CO• in the dark at 5 K was found only in Xe. This finding represents a prominent example of matrix-dependent chemical dynamics, which may involve the tunnelling mechanism. Probable mechanisms of acetaldehyde radiolysis and their implications for astrochemistry, atmospheric chemistry and low-temperature chemistry are discussed.