Kinetics of Hydrogen Abstraction Reactions from Fluoromethanes and Fluoroethanes

Abstract

Abstract Hydrogen abstraction reactions from hydrofluorocarbons are important reaction steps in both atmospheric and combustion chemistry. In this study, kinetics of the H-abstraction reactions from methane, ethane, fluoromethanes, and fluoroethanes by H, O, and OH radicals are investigated using the CBS-QB3//ωB97X-D quantum chemical method and transition state theory, and the site-specific reactivity of the fluoroalkanes systematically examined. The calculated rate constants for these reactions are found to accurately reproduce available experimental data for a wide temperature range, with only minor differences between experimental and theoretical barrier heights. Analysis of site-specific reactivity indicates that fluorine substitution at the β-carbon site of the fluoroalkanes systematically increases the barrier heights of H-abstraction, and the substitution effect at the α-carbon site can be interpreted by electron-withdrawing and steric effects. Fluorine substitution also decreases the pre-exponential factor for the H-abstraction reactions by O and OH radicals due to steric and electrostatic interactions between these radicals and the F atoms.