Unimolecular Reactions of CH₂BrCH₂Br, CH₂BrCH₂Cl, and CH₂BrCD₂Cl: Identification of the Cl−Br Interchange Reaction

Abstract

The recombination reactions of CH₂Br and CH₂Cl radicals have been used to generate vibrationally excited CH₂BrCH₂Br and CH₂BrCH₂Cl molecules with 91 kcal mol⁻¹ of energy in a room-temperature bath gas. The experimental unimolecular rate constants for elimination of HBr and HCl were compared to calculated statistical rate constants to assign threshold energies of 58 kcal mol⁻¹ for HBr elimination from C₂H₄Br₂ and 58 and 60 kcal mol⁻¹, respectively, for HBr and HCl elimination from C₂H₄BrCl. The Br−Cl interchange reaction was demonstrated and characterized by studying the CH₂BrCD₂Cl system generated by the recombination of CH₂Br and CD₂Cl radicals. The interchange reaction was identified from the elimination of HBr and DCl from CH₂ClCD₂Br. The interchange reaction rate is much faster than the rates of either DBr or HCl elimination from CH₂BrCD₂Cl, and a threshold energy of ≅43 kcal mol⁻¹ was assigned to the interchange reaction. The statistical rate constants were calculated from models of the transition states that were obtained from density functional theory using the B3PW91 method with the 6-31G(d′,p′) basis set. The model for HBr elimination was tested versus published thermal and chemical activation data for C₂H₅Br. A comparison of Br−Cl interchange with the Cl−F and Br−F interchange reactions in 1,2-haloalkanes is presented.