KINETIC AND MECHANISTIC STUDY OF THE REACTION OF ATOMIC CHLORINE WITHMETHYL-BROMIDE OVER AN EXTENDED TEMPERATURE-RANGE

A laser flash photolysis-resonance fluorescence technique has been emp loyed to study the kinetics of the reaction of chlorine atoms with met hyl bromide as a function of temperature (161-697 It) and pressure (20 -250 Torr) in nitrogen buffer gas. At T greater than or equal to 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 express ion adequately describes all kinetic data obtained: k(1a) = 1.02 x 10( -15)T(1.42) exp(- 605/T) cm(3) molecule(-1) s(-1). At temperatures in the range 161-177 K, reversible addition of Cl(P-2(J)) to CH3Br is obs erved, thus allowing rate coefficients and equilibrium constants for C H3BrCl formation and dissociation to be determined. Second- and third- law analyses of the equilibrium data lead to the following thermochemi cal parameters for the association reaction (1d): Delta H(298)degrees = -25.6 +/- 2.3 kJ mol(-1), Delta H(0)degrees = -24.0 +/- 2.9 kJ mol(- 1), Delta S(298K)degrees = -72.3 +/- 11.8 J K-1 mol(-1). In conjunctio n with the well-known heats of formation of Cl(P-2(J)) and CH3Br, the above Delta H values lead to the following heats of formation for CH3B rCl at 298 and OK: Delta H(f,298)degrees = 57.6 +/- 2.4 kJ mol(-1) and Delta H(f,0)degrees = 72.9 +/- 3.0 kJ mol(-1). Ab initio calculations using density functional theory and G2 theory reproduce the experimen tal bond strength reasonably well. The DFT calculations predict a CH3B rCl structure (used in the third-law analysis) where the C-Br-CI bond angle is 90 degrees and the methyl group adopts a staggered orientatio n with a pronounced tilt toward chlorine. Ab-initio calculations are a lso reported which examine the structures and energetics of adducts fo rmed 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 b eing the most strongly bound and the OH-adduct being the least strongl y bound. Bonding in CH3Br-X (X = F, Cl, OH) is discussed as are the im plications of the new experimental and theoretical results for atmosph eric chemistry. (C) 1998 Elsevier Science B.V. All rights reserved.