Kinetic and mechanistic study of the reaction of atomic chlorine with methyl bromide over an extended temperature range
Department of Chemistry, Auburn University, Auburn, AL 36849, USA Chemical Physics
(Impact Factor: 1.65).
06/1998; 231(2-3):155-169. DOI: 10.1016/S0301-0104(97)00356-X
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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- "Table 1.1 Comparison of Adduct Stabilities Dissociation Enthalpy at 298 K (kJ mol -1 ) a Adduct Experimental Theoretical SCS–Cl 40 b 33 c SCS–OH 41 d , 46 e , 44 f 27 g (CH 3 ) 2 S–Cl na 53 h , 81 i , 97 j , 74 k (CH 3 ) 2 S–Br 61 l , 51 m 59 k,m (CH 3 ) 2 S–OH 42 n , 45 o 50 p (CH 3 ) 2 (O)S–Cl 73 q 67 q , 73 r CH 3 I–Cl 54 s 44 s , 61 s , 59 t , 52 u C 2 H 5 I–Cl 58 v 38 v , 59 v , 62 w a Uncertainties in theoretical values are typically ~8 kJ mol -1 . b Nicovich et al., 1990; c Wang and Phillips, 2002; d Hynes et al., 1988; e Murrels et al., 1990; f Diau and Lee, 1991; g McKee and Wine, 2001; h Resende and De Almeida, 1997; i Wilson and Hirst, 1997; j Thompson et al., 2002; k Enami et al., 2004; l Wine et al., 1993; m Nakano et al., 2001; n Hynes et al., 1995; o Barone et al., 1996; p McKee, 2003; q Nicovich et al., 2006; r Vandresen and Resende, 2004; s Ahyens et al., 1997; t Enami et al., 2005; u Lazarou et al., 1997; v Orlando et al., 2005; w Piety et al., 1998. "
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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
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ABSTRACT: The reaction of Cl atoms with CH2ClI was studied in the gas phase with the very low pressure reactor (VLPR) technique over the temperature range 273−363 K. The absolute rate constant was found to be k1 = 3.13 ± 0.27 × 10-11 cm3 molecule-1 s-1 (2σ uncertainty), independent of temperature. The reaction occurs through iodine atom abstraction leading to CH2Cl and ICl products. The secondary reaction of Cl atoms with ICl as well as the self-reaction of CH2Cl radicals was also occurring, leading to the production of I atoms, Cl2, CH2CHCl, and HCl, respectively. Ab initio calculations at the MP2/3-21++G(2d,2p) level of theory suggest that the title reaction may proceed via the intermediate formation of a weakly bound adduct CH2ClI−Cl, with an I−Cl bond strength of 54.8 kJ mol-1. The enthalpies of formation for CH2ClI and CHClI at 298.15 K were also calculated at the MP2/3-21++G(2d,2p) level of theory to be 13.0 and 212.0 kJ mol-1, respectively.
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ABSTRACT: A laser flash photolysis-resonance fluorescence technique has been employed to investigate the kinetics of reactions of the important stratospheric species bromine nitrate (BrONO2) with ground-state atomic bromine (k(1)), chlorine (k(2)), and oxygen (k(3)) as a function of temperature (224-352 K) and pressure (16-250 Torr of N-2). The rate coefficients for all three reactions are found to be independent of pressure and to increase with decreasing temperature. The following Arrhenius expressions adequately describe the observed temperature dependencies (units are 10(-11) cm(3)molecule(-1)s(-1)): k(1) = 1.78 exp(365/T), k(2) = 6.28 exp(215/T), and k(3) = 1.91 exp(215/T). The accuracy of reported rate coefficients is estimated to be 15-25% depending on the magnitude of the rate coefficient and on the temperature. Reaction with atomic oxygen is an important stratospheric loss process for bromine nitrate at altitudes above similar to 25 km; this reaction should be included in models of stratospheric chemistry if bromine partitioning is to be correctly simulated in the 25-35 km altitude regime.
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