Radiation-induced chemistry in the C2H2–H2O system at cryogenic temperatures: A matrix isolation study

Investigations of the low-temperature radiation-induced transformations in the C2H2–H2O system are relevant to the chemistry occurring in interstellar and cometary ices. In this work we applied a matrix isolation technique to study the radiation-driven evolution of this system at molecular level in order to get new mechanistic insight. The 1:1 C2H2∙∙∙H2O complexes were prepared in various solid noble-gas matrices (Ar, Kr, Xe) and these icy matrices were subjected to X-ray irradiation at 5 K. Decomposition of initial complex and formation of products were monitored by FTIR spectroscopy. It was found that complexation with H2O resulted in significant enhancement of the radiolytic decay of C2H2 molecules and provided new channels for its radiation-induced transformations. Ketene (both isolated H2CCO and in the form of H2CCO–H2 pair), ketenyl radical (HCCO), carbon monoxide (CO) and methane (CH4) were observed as main products of the C2H2∙∙∙H2O radiolysis. In addition, vinyl alcohol (CH2CHOH) was detected in an Ar matrix. The mechanistic interpretation (reaction routes leading to formation of these products) is discussed on the basis of consideration of kinetic dependencies and matrix effects. Conversion of C2H2∙∙∙H2O to CH4 is a prominent example of water-mediated cleavage of the C≡C triple bond, which may occur under prolonged irradiation in rigid media. Possible astrochemical implications of the obtained results are highlighted.