Kinetic and mechanistic study of the reaction of atomic chlorine with methyl bromide over an extended temperature range Original Research Article

A laser flash photolysis–resonance fluorescence technique has been employed to study the kinetics of the reaction of chlorine atoms with methyl bromide as a function of temperature (161–697 K) and pressure (20–250 Torr) in nitrogen buffer gas. At T≥213 K, where information available in the literature suggests that hydrogen transfer is the dominant reaction pathway, observed rate coefficients are pressure independent and the following modified Arrhenius expression adequately describes all kinetic data obtained: k1a=1.02×10−15T1.42 exp(−605/T) cm3 molecule−1 s−1. At temperatures in the range 161–177 K, reversible addition of Cl(2PJ) to CH3Br is observed, thus allowing rate coefficients and equilibrium constants for CH3BrCl formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction (1d): ΔH298o=−25.6±2.3 kJ mol−1, ΔH0o=−24.0±2.9 kJ mol−1, ΔS298 Ko=−72.3±11.8 J K−1 mol−1. In conjunction with the well-known heats of formation of Cl(2PJ) and CH3Br, the above ΔH values lead to the following heats of formation for CH3BrCl at 298 and 0 K: ΔHf, 298o=57.6±2.4 kJ mol−1 and ΔHf, 0o=72.9±3.0 kJ mol−1. Ab initio calculations using density functional theory and G2 theory reproduce the experimental bond strength reasonably well. The DFT calculations predict a CH3BrCl structure (used in the third-law analysis) where the C–Br–Cl bond angle is 90° and the methyl group adopts a staggered orientation with a pronounced tilt toward chlorine. Ab-initio calculations are also reported which examine the structures and energetics of adducts formed from addition of F atoms and OH radicals to CH3Br. Structures of CH3BrF and CH3BrOH are similar to that of CH3BrCl, with the F-adduct being the most strongly bound and the OH-adduct being the least strongly bound. Bonding in CH3Br–X (X=F, Cl, OH) is discussed as are the implications of the new experimental and theoretical results for atmospheric chemistry.