Document Detail

Kinetics of Elementary Steps in the Reactions of Atomic Bromine with Isoprene and 1,3-Butadiene Under Atmospheric Conditions.
MedLine Citation:
PMID:  22435953     Owner:  NLM     Status:  Publisher    
Laser flash photolysis of CF2Br2 has been coupled with time-resolved detection of atomic bromine by resonance fluorescence spectroscopy to investigate the gas-phase kinetics of early elementary steps in the Br-initiated oxidations of isoprene (2-methyl-1,3-butadiene, Iso) and 1,3-butadiene (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 K 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(1b) = -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) 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 to 700 Torr) whereas k4 increases from 3.2 to 4.7 × 10-13 cm3 molecule-1 s-1 as 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, while 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.
Patrick Laine; Yoon S Sohn; John Michael Nicovich; Michael L McKee; Paul H Wine
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-3-21
Journal Detail:
Title:  The journal of physical chemistry. A     Volume:  -     ISSN:  1520-5215     ISO Abbreviation:  -     Publication Date:  2012 Mar 
Date Detail:
Created Date:  2012-3-22     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9890903     Medline TA:  J Phys Chem A     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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