Kinetic hydrogen isotope effect in the fluorination of H2, CH4 and CHCl3

Abstract

Kinetic isotope effects for the hydrogen abstraction reactions of fluorine atoms with H₂, CH₄ and CHCl₃ have been measured by indirect competitive methods. The relative rate constants are given by k(CHCl₃)/k(CDCl₃)= 0.81 ± 0.25 exp[(620 ± 50)/RT], k(H₂)/k(D₂)= 1.5 ± 0.4 exp[(130 ± 300)/RT], k(CH₄)/k(CD₄)= 1.0 ± 0.3 exp[(230 ± 200)/RT]. The result for k(H₂)/k(D₂) agrees with that previously measured. These relatively small kinetic isotope effects are compared with rate parameters calculated from activated complex theory and classical trajectory analysis. Hydrogen atom transfer by quantum mechanical tunnelling is apparently unimportant.