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...

The family of isomeric H2C3O+• radical cations is of great interest for physical organic chemistry and chemistry occurring in extraterrestrial media. In this work, we have experimentally examined a unique synthetic route to the generation of H2C3O+• from the C2H2···CO intermolecular complex and also considered the relative stability and monomolecular transformations of the H2C3O+• isomers through high-level ab initio calculations. The structures, energetics, harmonic frequencies, hyperfine coupling constants, and isomerization pathways for several of the most important H2C3O+• isomers were calculated at the UCCSD­(T) level of theory. The complementary FTIR and EPR studies in argon matrices at 5 K have demonstrated that the ionized C2H2···CO complex transforms into the E-HCCHCO+• isomer, and this latter species is supposed to be the key intermediate in further chemical transformations, providing a remarkable piece of evidence for kinetic control in low-temperature chemistry. Photolysis of this species at λ = 410–465 nm results in its transformation to the thermodynamically most stable H2CCCO+• isomer. Possible implications of the results and potentiality of the proposed synthetic strategy to the preparation of highly reactive organic radical cations are discussed.

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.

In this work we have examined the radiation-induced synthetic chemistry occurring in an astrochemically important C2H2–CO system at the molecular level using a matrix isolation approach. The 1:1 C2H2∙∙∙CO intermolecular complex of linear structure was obtained in the solid low-temperature (5 K) noble gas matrices by deposition of the C2H2/CO/Ng (Ng = Ar, Kr, Xe) gaseous mixtures and characterized by Fourier-transform infrared (FTIR) spectroscopy. It was found that the X-ray radiolysis of the C2H2∙∙∙CO complex resulted in formation of C3O (tricarbon monoxide), HCCCHO (propynal), c-H2C3O (cyclopropenone), H2CCCO (propadienone), and HC3O (oxoprorynyl radical). This means that the studied complex may be considered as the simplest building block (or minimal size of intermolecular reactor) for cold astrochemistry occurring in mixed interstellar ices. Remarkably, the discovered transformations of the complex actually represent synthetic routes leading to various C3 species, whereas the acetylenic C–H bond cleavage yielding ethynyl radical appears to be a minor process. Prolonged irradiation results in dehydrogenation, while the C3 skeleton is retained. The interpretation of the C2H2∙∙∙CO radiolysis mechanism (possible reactions pathways) is provided based on the analysis of kinetic curves and matrix effect. Astrochemical implications of the results are discussed.

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...