The reactions CH+H2rarr;thinsp;k1products and CD+D2rarr;thinsp;k2products were studied using a laser photolysis/laserhyphen;induced fluorescence (LIF) technique. The roomhyphen;temperature rate constant for reaction (1) was found to be pressure dependent between 25 and 600 Torr. Reactions (1) and (2) were studied as a function of temperature in the range 159ndash;658 K at a total pressure of 100 Torr. Two pathways dominate these reactions. Below 400 K, addition followed by collisional stabilization of the adduct predominates. The rate constants display negative temperature dependences and are described byk1=(2.37plusmn;0.43)times;10minus;12thinsp;explsqb;(1041plusmn;85)/RTrsqb; andk2=(3.51plusmn;0.58)times;10minus;12thinsp;explsqb;(1030plusmn;90)/RTrsqb; cm3thinsp;moleculeminus;1thinsp;sminus;1withEain units of calthinsp;molminus;1. Above 500 K, production of CH2+H or CD2+D with an activation energy of 4plusmn;2 kcalthinsp;molminus;1dominates. In the reactions of CH+D2and CD+H2, anomalously fast removal of the parent radical is observed because of formation and dissociation of chemically activated addition complexes which lead to isotopically substituted parent radicals. LIF spectra of these product radicals were obtained. A transitionhyphen;statehyphen;theory model was applied to a mechanism in which collision stabilization and product formation proceed through a single complex.
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