Kinetic isotope effects (KIEs) in the free radical reactions-application to the atmospheric chemistry

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Stable isotope analysis of atmospheric trace gases can be very useful in improving our understanding of their sources, sinks, and transport. The rationale is that various sources have characteristic isotopic ratios and that sink processes are accompanied by isotope fractionation. The last few years have witnessed great concern about changes in the chemical composition of the Earth's atmosphere, and a significant amount of research has been devoted to this questions. One of the principal subjects of the recent studies is methane. Methane is an important greenhouse gas whose relative concentrations in the stratosphere and troposphere is affected by reaction with the radicals OH, O (^1D), and Cl. A very useful indicator of the sources and sinks of methane in the environment is provided by measurements of the ^<12>C/^<13>C ratios, and interpretation of these data requires an understanding of the kinetic isotope effects (KIEs) of several processes that lead to fractionation. Kinetic isotope effects (KIEs) are rats of ratios of rate constants for pains of isotopically different molecules. They are widely used as tools for elucidating reaction mechanism and have been investigated both experimentally and theoretically. In this project a new method to determine the KIEs value of free radical reactions has been developed by using the high sensitive photoionization mass spectrometer. The KIEs values for reactions of methyl radicals (^<12>CH_3, ^<12>CH_3, CD_3) with atomic or molecular oxygen were determined.