Kinetics of Elementary Steps in the Reactions of Atomic Bromine with Isoprene and 1,3-Butadiene under Atmospheric Conditions

Abstract

Laser flash photolysis of CF(2)Br(2) 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: k(1a)(T) = (1.22 ± 0.57) × 10(-11) exp[(-2100 ± 280)/T] cm(3) 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 (k(1b), k(-1b)) and Br + Bu ↔ Br-Bu (k(3b), k(-3b)) is observed. Analysis of the approach to equilibrium data allows the temperature- and pressure-dependent rate coefficients k(1b), k(-1b), k(3b), 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 k(1b)/k(-1b) and k(3b)/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 Δ(r)H and J mol(-1) K(-1) for Δ(r)S; uncertainties are accuracy estimates at the 95% confidence level): Δ(r)H(0)(1b) = -66.6 ± 7.1, Δ(r)H(298)(1b) = -67.5 ± 6.6, and Δ(r)S(298)(3b) = -93 ± 16; Δ(r)H(0)(3b) = -62.4 ± 9.0, Δ(r)H(298)(3b) = -64.5 ± 8.5, and Δ(r)S(298)(3b) = -94 ± 20. Examination of the effect of added O(2) on Br kinetics under conditions where reversible adduct formation is observed allows the rate coefficients for the Br-Iso + O(2) (k(2)) and Br-Bu + O(2) (k(4)) reactions to be determined. At 298 K, we find that k(2) = (3.2 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1) independent of pressure (uncertainty is 2σ, precision only; pressure range is 25-700 Torr) whereas k(4) increases from 3.2 to 4.7 × 10(-13) cm(3) 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 O(2) 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 conditio...

Closing the Shell: Gas‐Phase Solvation of Halides by 1,3‐Butadiene

“…Despite the increase in complex stability in these cases, the 7 cis structure still retains the imaginary mode (132i cm −1 ) and it is not significantly stabilised with respect to the butadiene monomer. The 7 trans structure however is reminiscent of the prereaction adduct in radical activation of butadiene by bromine to produce peroxy radicals, as reported in the literature [9] . In both cases similarly to the inversion of halide/halogen stability in the propene complexes, the halogen‐butadiene complexes are less stable than the corresponding propene complexes.…”

“…The 7trans structure however is reminiscent of the prereaction adduct in radical activation of butadiene by bromine to produce peroxy radicals, as reported in the literature. [9] In both cases similarly to the inversion of halide/ halogen stability in the propene complexes, the halogenbutadiene complexes are less stable than the corresponding propene complexes. This is likely due to a combination of delocalisation through the carbon chain reducing the bonding interaction at any given carbon.…”

“…The rate constant for this reaction is then 4.7×10 −13 cm −3 molecule −1 s −1 at 298 K and 700 Torr [9] . For the chlorinated analogue, the stronger Cl–H interaction is believed to favour hydrogen abstraction reactions and the formation of corresponding radicals.…”

“…The rate constant for this reaction is then 4.7 × 10 À 13 cm À 3 molecule À 1 s À 1 at 298 K and 700 Torr. [9] For the chlorinated analogue, the stronger Cl-H interaction is believed to favour hydrogen abstraction reactions and the formation of corresponding radicals. The experimentally measured rate constant for the reaction of chlorine atoms with butadiene here is 3.31 × 10 À 10 cm À 3 molecule À 1 s À 1 .…”