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

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA; School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA; Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332-0865, USA; Department of Chemistry, Auburn University, Auburn, AL 36849, USA
Chemical Physics (Impact Factor: 1.96). 01/1998; DOI: 10.1016/S0301-0104(97)00356-X

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.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A number of weakly bound adducts play important roles in atmospheric chemistry, such as DMS OH and CS2 OH. The work comprising this dissertation involves kinetic and spectroscopic studies of adducts formed between halogen atoms and the important atmospheric trace gases CS2, CH3SCH3 (DMS), CH3I, and C2H5I. The results reported in these studies are useful for developing an understanding of the reactivity of these species and for testing the ability of electronic structure theory and reaction rate theory to predict or rationalize any observed trends. Oxidative pathways of both alkyl halides and sulfur compounds, especially DMS, are of atmospheric interest based on the roles of these species in affecting the oxidizing capacity of the troposphere and in the formation of new particles which impact the Earth s radiation budget and climate variability. The experimental approach employed laser flash photolysis (LFP) coupled with time resolved UV-visible absorption spectroscopy (TRUVVAS) to investigate the spectroscopy and kinetics of the gas phase adducts: SCS Cl, CH3I Cl, C2H5I Cl, (CH3)2S Br, and (CH3)2S I. Ph.D. Committee Chair: Wine, Paul; Committee Member: Huey, Greg; Committee Member: Nenes, Athanasios; Committee Member: Weber, Rodney; Committee Member: Whetten, Robert
  • [Show abstract] [Hide abstract]
    ABSTRACT: A laser flash photolysis–resonance fluorescence technique has been employed to study the kinetics of the reactions of atomic chlorine with acetone (CH3C(O)CH3; k1), 2-butanone (C2H5C(O)CH3; k2), and 3-pentanone (C2H5C(O)C2H5; k3) as a function of temperature (210–440 K) and pressure (30–300 Torr N2). No significant pressure dependence is observed for any of the reactions studied. Arrhenius expressions (units are 10−11 cm3 molecule−1 s−1) obtained from the data are k1(T) = (1.53 ± 0.19) exp[(−594 ± 33)/T], k2(T) = (2.77 ± 0.33) exp[(+76 ± 33)/T], and k3(T) = (5.66 ± 0.41) exp[(+87 ± 22)/T], where uncertainties are 2σ and represent precision only. The accuracy of reported rate coefficients is estimated to be ±15% over the entire range of pressure and temperature investigated. The room temperature rate coefficients reported in this study are in good agreement with a majority of literature values. However, the activation energies reported in this study are in poor agreement with the literature values, particularly for 2-butanone and 3-pentanone. Possible explanations for discrepancies in published kinetic parameters are proposed, and the potential role of Cl + ketone reactions in atmospheric chemistry is discussed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 259–267, 2008
    International Journal of Chemical Kinetics 01/2008; 40(5):259-267. · 1.19 Impact Factor
  • [Show abstract] [Hide abstract]
    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