ИСТИНА

Model matrix-isolation studies on the radiation chemistry of astrochemically relevant molecules are very useful for a better understanding of the radiation chemistry occurring in cold space media. Acetylene (C2H2) is one of the simple molecules known to occur in various extraterrestrial environments (particularly, in cometary ices), mainly as an admixture to principal components, such as CO. Thus, one may expect that the acetylene complex with CO may be considered as an important building block for complex organic molecules and biomolecules in space. In a wider context, the impact of weak intermolecular complexes may present general interest for basic radiation chemistry. In this work, we have examined the low-temperature radiation-induced transformations of the 1:1 C2H2···CO intermolecular complex using matrix-isolation approach using FTIR and EPR spectroscopy complemented with theoretical studies at the UCCSD(T) level of theory. Matrix samples were obtained by deposition of gaseous mixtures (C2H2/CO/Ng 1/3/1000; Ng = Ar, Kr, Xe) onto a cold KBr substrate mounted in a closed-cycle helium cryostat. The conditions of the deposition procedure were optimized to obtain a sufficient amount of the precursor 1:1 C2H2∙∙∙CO complex. The deposited matrices were irradiated with X-rays (effective energy ca. 20 keV) to different doses (up to 200 kGy) at 5 K. Photolysis of the irradiated samples was performed using appropriate LEDs (λD = 465 nm and λD = 410 nm). The radiation-induced products and intermediates were characterized by FTIR and EPR spectroscopy. It was found that the X-ray radiolysis of the low-temperature noble gas matrices containing 1:1 C2H2∙∙∙CO complex resulted in the formation of C3O (tricarbon monoxide), HCCCHO(propynal), c-H2C3O (cyclopropenone), H2CCCO (propadienone), and HC3O (oxoprorynyl radical) at 5 K. Remarkably, the observed transformations of the complex 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. IR and EPR spectroscopic studies supported by extensive ab initio UCCSD(T) calculations revealed the formation of a particular isomer of the H2C3O+ radical cation as a key reactive intermediate. The interpretation of the C2H2∙∙∙CO radiolysis mechanism(possible reactions pathways) is given based on the analysis of kinetic curves and matrix effect.