Isotope effect in thermal ionic reactions of isomeric transition-activated 80Br and 82Br in the HBr-CH4 system.

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The reactions of isomeric-transition activated 82Br were studied in the gaseous H82mBr–CH4(HBr/CH4=0.1±0.01) system. CH382Br and CH282BrBr were identified as the reaction products; their absolute yields were 0.8% for CH382Br and 3.7% for CH282BrBr respectively. The results were compared with those obtained in the H80mBr–CH4 and 80mBr2 (or 82mBr2)–CH4 systems. The effect of Krand Xe additives showed that these products were formed only through kinetic energy-independent (thermal-ionic) reactions, as opposed to the results obtained in the 80mBr2 (or 82mBr2)–CH4 system. This difference was explained on the basis of the kinetic energies initially acquired by the bromine atoms in the molecular-explosion process. In addition, an outstanding isotope effect resulting from the difference in the decay schemes of 80mBr and 82mBr was observed between the I. T.-activated 80Br and 82Br reactions in the H80mBr–CH4 and H82mBr–CH4 systems, and it was pointed out from the analysis of the 80mBr-data that the two successive internal conversions in the 80mBr decay were responsible for this isotope effect. It was considered that the second internal conversion started in a liquid-like condition such as ion-clusters containing the 80Br in the intermediate state; this idea is supported by the experimental results obtained in the condensed phase.
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