ChemInform Abstract: Laser Photolysis/Laser‐Induced Fluorescence Studies of Reaction Rates of OH with CH3Cl, CH2Cl2, and CHCl3 Over an Extended Temperature Range

Abstract

A modified laser photolysis/laser-induced fluorescence technique has been used to measure atmospheric pressure absolute rate coefficients for the reaction of hydroxyl (OH) radicals with the chlorinated methanes (CH3Cl, CH2Cl2, and CHCl3). Data have been obtained for these compounds over the widest temperature range (292–800 K) that has been reported in the literature using a single experimental apparatus. The temperature dependence of the rate data is best represented by the following three-parameter expressions: Uncertainties in the pre-exponential and exponential term are expressed as 95% confidence intervals. For the temperature exponent, error limits represent a ±10% change in the total error of best fit. The degree of curvature in the Arrhenius plots appeared to increase with increasing Cl substitution of the reactant. However, the uncertainty in the temperature exponent for the CH3Cl data was large in comparison with the other chlorinated methanes. Thus, data of greater precision at elevated temperatures are necessary to further explore this relationship. The rate coefficients were compared with recent semiempirical and transition state theory models for haloalkane-OH hydrogen transfer reactions over a temperature range of 250–800 K. The transition state model of Cohen and Benson was in excellent agreement with the CH3Cl and CH2Cl2 data. The semiempirical structure activity relationship developed by Atkinson represented the best fit of the CHCl3 data, although it underestimated the experimental data by more than a factor of 2 at 800 K. The extreme care used to remove and alayze for reactive impurities along with the agreement with other experimental studies suggests that transition state and semi-empirical models for CHCl3 must be modified to account its reaction behavior at high temperature.