PNNL

Abstract

Previously measured Cl dissociation rate constants for CCl4 and CFCl3 were analyzed with three different kinetics modeling calculations. The three models differ in detail but primarily are distinguished by the manner in which the high-pressure limiting rate constant is determined:? model 1 involves a calibration to transport properties of the dissociated fragments, model 2 uses a Gorin model with a hindrance parameter, and model 3 requires variational transition state theory on an ab initio reaction path where all low-frequency motion off the path is presumed to be a free rotation. All three models have two adjustable parameters:? the dissociation energy E0 and the average energy transferred to the buffer gas down. All three models are found to give comparable fits to the experiment and produce quite similar values for the adjustable parameters. For CCl4, the values are E0 = (68.2 ± 1.2) kcal mol-1 and down = (750 ± 125) cm-1. For CFCl3, the values are E0 = (76.5 ± 0.5) kcal mol-1 with down = (800 ± 215) cm-1. These values are compared to those obtained in similar studies for CF2Cl2 and CF3Cl. The results indicate a substantial and consistent decrease in the C-Cl bond energy with each additional chlorine substitution in the chlorofluoromethanes. Isodesmic electronic structure calculations at the MP2 level confirm this effect but find it to be a little smaller than the experimental results indicate. Extended electronic structure calculations provide heats of formation for all nine CHxFyClz methyl radicals.