Kinetics of Hydrogen Abstraction Reactions from Fluoromethanes and Fluoroethanes

• Published in: Bulletin of the Chemical Society of Japan Vol. 87; no. 8; pp. 890 - 901
• Main Authors: Matsugi, Akira, Shiina, Hiroumi
• Format: Journal Article
• Language: English
• Published: Tokyo The Chemical Society of Japan 15.08.2014; Chemical Society of Japan
• Subjects: Barriers; Fluorine; Fluorometers; Hydrogen; Methane; Radicals; Reaction kinetics; Steric effects
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.20140101

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.