Kinetic and mechanistic study of the reaction of atomic chlorine with methyl bromide over an extended temperature range
ABSTRACT 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.
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ABSTRACT: We report electronically nonadiabatic dynamics calculations including spin-orbit coupling for the photodissociation of CH(2)ClBr to yield Cl((2)P(3∕2)), Cl((2)P(1∕2)), Br((2)P(3∕2)), and Br((2)P(1∕2)). The potential energy is a 24 × 24 matrix (divided up here into four 6 × 6 blocks in a first approximation to the problem), in a spin-coupled fully diabatic representation obtained by combining the spin-free fourfold way with single-center spin-orbit coupling constants. The spin-free calculations are carried out by multiconfiguration quasidegenerate perturbation theory, and the fully diabatic potentials including spin-orbit coupling are fit to a matrix reactive force field. The dynamics are carried out by the coherent switches with decay of mixing method in the diabatic representation. The results show qualitative agreement with experiment.The Journal of Chemical Physics 09/2012; 137(10):10A501. · 3.12 Impact Factor
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ABSTRACT: The kinetics of the gas-phase reactions of chlorine atoms with bromomethane CH3Br and D-bromomethane CD3Br was studied experimentally. The relative rate method was applied using Cl+CH4 as the reference reaction. The rate constants for H-abstraction from CH3Br (kH) and D-abstraction from CD3Br (kD) were measured in the temperature range of 298–527K and at a total pressure of 100Torr. The temperature dependencies of the rate constants are described by the expressions: kH=(1.43±0.22)×10−11×exp(−1040±30/T) and kD=(1.36±0.31)×10−11×exp(−1595±30/T)cm3molecule−1s−1. The kinetic isotope effect, described by the ratio kH/kD, was found to be (1.07±0.14)×exp(555±20/T).Chemical Physics Letters 01/2010; 486:7-11. · 2.15 Impact Factor
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ABSTRACT: The kinetics of the gas-phase reactions of chlorine atoms with dichloromethane (CH2Cl2) and D-dichloromethane (CD2Cl2) was studied using the relative rate method with Cl+CH3Br as the reference reaction. The rate constants for H-abstraction from CH2Cl2 (kH) and D-abstraction from CD2Cl2 (kD) were measured in the temperature range of 298–527K and at a total pressure of 100Torr using N2 as a diluent. The temperature dependencies of the rate constants (with the 2σ error limits) are described by the expressions: kH=(8.69±0.82)×10−12×exp(−955±20/T) and kD=(6.98±0.91)×10−12×exp(−1285±25/T)cm3molecule−1s−1. The kinetic isotope effect, described by the ratio kH/kD, was found of 3.8±0.2 at room temperature.Chemical Physics Letters 01/2011; 514(4):220-225. · 2.15 Impact Factor