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

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 10 calculated statistical rate constants to assign threshold energies of 58 kcal mol(-1) for HBr elimination from C2H4Br, and 58 and 60 kcal mol(-1), respectively, for HBr and HCl elimination front 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 congruent to 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.