Laser studies of methyl radical reactions with Cl2 and Br2: Absolute rate constants, product vibrational excitation, and hot radical reactions

Abstract

Reactions of methyl radicals with Cl2 and Br2 are studied by pulsed laser dissociation of CH3I followed by time-resolved detection of infrared vibrational fluorescence from the C–H stretch modes of the methyl halide product. This method provides a determination of the absolute rate constants for the methyl radical reactions. The rate constants for thermal reactions of CH3 with Cl2 and Br2 are 1.5(±0.1)×10−12 and 2.0(±0.4)×10−11 cm3 molecule−1 s−1, respectively. From the fluorescence intensity of the methyl halide product, an estimate can be made of the fraction of energy available upon reaction that goes into product vibration. For both reactions this is determined to be about 0.5. In addition, the reaction rates are observed to be considerably enhanced for hot methyl radicals produced in the dissociation of CH3I. The enhancement is shown to be due predominantly to the translational excitation, as opposed to the vibrational excitation, imparted in the photofragmentation.