Kinetics of Elementary Steps in the Reactions of Atomic Bromine with Isoprene...

Laine, P. L., Y. S. Sohn, J. M. Nicovich, M. L. McKee, and P. Wine (2012), Kinetics of Elementary Steps in the Reactions of Atomic Bromine with Isoprene and 1,3-Butadiene under Atmospheric Conditions, J. Phys. Chem. A, 116, 6341-6357, doi:10.1021/jp212127v.

Laser flash photolysis of CF2Br2 has been coupled with time-resolved detection of atomic bromine by resonance fluorescence spectroscopy to investigate the gasphase kinetics of early elementary steps in the Br-initiated oxidations of isoprene (2-methyl-1,3-butadiene, Iso) and 1,3butadiene (Bu) under atmospheric conditions. At T ≥ 526 K, measured rate coefficients for Br + isoprene are independent of pressure, suggesting that hydrogen transfer (1a) is the dominant reaction pathway. The following Arrhenius expression adequately describes all kinetic data at 526 K ≤ T ≤ 673 K: k1a(T) = (1.22 ± 0.57) × 10−11 exp[(−2100 ± 280)/T] cm3 molecule−1 s−1 (uncertainties are 2σ and represent precision of the Arrhenius parameters). At 271 K ≤ T ≤ 357 K, kinetic evidence for the reversible addition reactions Br + Iso ↔ Br−Iso (k1b, k−1b) and Br + Bu ↔ Br−Bu (k3b, k−3b) is observed. Analysis of the approach to equilibrium data allows the temperature- and pressure-dependent rate coefficients k1b, k−1b, k3b, and k−3b to be evaluated. At atmospheric pressure, addition of Br to each conjugated diene occurs with a near-gas-kinetic rate coefficient. Equilibrium constants for the addition/dissociation reactions are obtained from k1b/k−1b and k3b/k−3b, respectively. Combining the experimental equilibrium data with electronic structure calculations allows both second- and third-law analyses of thermochemistry to be carried out. The following thermochemical parameters for the addition reactions 1b and 3b at 0 and 298 K are obtained (units are kJ mol−1 for ΔrH and J mol−1 K−1 for ΔrS; uncertainties are accuracy estimates at the 95% confidence level): ΔrH0(1b) = −66.6 ± 7.1, ΔrH298(1b) = −67.5 ± 6.6, and ΔrS298(3b) = −93 ± 16; ΔrH0(3b) = −62.4 ± 9.0, ΔrH298(3b) = −64.5 ± 8.5, and ΔrS298(3b) = −94 ± 20. Examination of the effect of added O2 on Br kinetics under conditions where reversible adduct formation is observed allows the rate coefficients for the Br−Iso + O2 (k2) and Br−Bu + O2 (k4) reactions to be determined. At 298 K, we find that k2 = (3.2 ± 1.0) × 10−13 cm3 molecule−1 s−1 independent of pressure (uncertainty is 2σ, precision only; pressure range is 25−700 Torr) whereas k4 increases from 3.2 to 4.7 × 10−13 cm3 molecule−1 s−1 as the pressure increases from 25 to 700 Torr. Our results suggest that under atmospheric conditions, Br−Iso and Br−Bu react with O2 to produce peroxy radicals considerably more rapidly than they undergo unimolecular decomposition. Hence, the very fast addition reactions appear to control the rates of Br-initiated formation of Br−Iso−OO and Br−Bu−OO radicals under atmospheric conditions. The peroxy radicals are relatively weakly bound, so conjugated diene regeneration via unimolecular decomposition reactions, though unimportant on the time scale of the reported experiments (milliseconds), is likely to compete effectively with bimolecular reactions of peroxy radicals under relatively warm atmospheric conditions as well as in 298 K competitive kinetics experiments carried out in large chambers.

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Tropospheric Composition Program (TCP)
Upper Atmosphere Research Program (UARP)