Radiation-induced transformations of acetylenic hydrocarbons in solid noble gas matrices

Complete text of publication follows. The photo- and radiation chemistry of acetylene in solid noble gas matrices has attracted interest in connection with synthesis of novel-type, unusual high-energy organic molecules and radicals, such as HNgCCH (Ng = Xe, Kr), HXeCCXeH, and HXeCC. A common way of preparation of such species involves dissociation of acetylene in a noble-gas matrix below 20 K followed by anneling-induced mobilization of H atoms. Quantum chemical calculations also predict stability of xenon derivatives of higher acetylenes (or even oligomers). However the mechanistic aspects of the radiation-chemical transformations are unclear. In this report we present the results of matrix-isolation studies of radiation-induced transformations of propyne and tert-butyl acetylene (3,3- dimethylpropyne-1) in solid Xe and Kr matrices using a combination of EPR and IR spectroscopy. The deposited matrix samples (ca. 1:1000) were irradiated with X-rays at 15 - 17 K and annealed at 30 - 32 K (krypton) or 40 - 45 K (in xenon). In the case of propyne, the dissociation of (C≡C)-H bond is quite efficient, however, the resulting radical·C≡CCH3 appears to be unstable at 16 K. In the case of t-butyl acetylene, we found EPR evidences for formation of methyl radicals, H atoms and R·CH2 type radicals originating from t-butyl group. The IR data reveal formation of methane directly after irradiation at 16 K. C-C bond rupture increases when turning from Xe to Kr matrix, which may be explained by effect of excess energy in positive hole or excitation transfer. We were unable to detect the products of Xe or Kr insertion to acetylenic moiety. We can conclude that alkyl substituent has crucial effect on the mechanism of radiation-induced transformations of acetylene derivatives in solid noble-gas matrices. Pathways and prospects of synthesis of organo-noble-gas compounds from various alkynes are discussed. The work was supported by INTAS (no 05-1000008-8017) and Russian Foundation for Basic Research (no. 09-03- 00848).