The kinetics and mechanisms of the reactions of O(3P) atoms with CH3CN and CF3CN

摘要

Absolute rate constants for the reactions of O(3P) atoms with CH3CN and CF3CN were measured using the flash photolysisndash;resonance fluorescence method under conditions which minimized complications from secondary reactions. These reactions were studied over the temperature range of 383ndash;500 K. The Arrhenius expression obtained for O(3P)+CH3CN was (7.27plusmn;1.75) times;10minus;13exp(minus;4770plusmn;200/RT) cm3moleculeminus;1sdot;secminus;1, while that for the O(3P)+CF3CN reaction was (14.2plusmn;5.2) times;10minus;13exp(minus;6130plusmn;310/RT) cm3moleculeminus;1sdot;secminus;1. Mechanistic information concerning these reactions was obtained from kinetic isotope effect studies using CD3CN in place of CH3CN and from fast flow tube studies of both the CH3CN and CF3CN reactions using direct mass spectrometric and ESR detection methods. The lack of an isotope effect in the CD3CN/CH3CN experiments coupled with the reaction products identified mass spectrometrically leads to the conclusion that the major reaction channel in both systems involves displacement of CH3and CF3by O(3P). In addition, there may be some contribution to the overall reaction from a direct molecular rearrangement for both O(3P)+CH3CN and O(3P)+CF3CN and from abstraction of H atom for O(3P)+CH3CN, the latter being more significant at higher temperatures. The kinetic parameters obtained in this work are compared with values from O(3P) reactions with other nitriles, alkynes, and olefins. In all three reaction classes, the major initial step is assumed to be interaction of the electrophilic O(3P) atom with the Pgr; bonds of the substrate molecules.