Rate coefficients for the reaction of OH radicals with cis-3-hexene: an experimental and theoretical study

Abstract

The kinetics of the cis-3-hexene + OH reaction were investigated by an experimental relative rate method and at the density functional theory level. The experimental set-up consisted of a 200 L Teflon bag, operated at atmospheric pressure and 298 K. OH radicals were produced by the photolysis of H₂O₂ at 254 nm. Relative rate coefficients were determined by comparing the decays of the cis-3-hexene and reference compounds (cyclohexene, 2-buten-1-ol and allyl ether). The mean second-order rate coefficient value found was (6.27 ± 0.66) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, the uncertainty being estimated by propagation of errors. Theoretical calculations for the addition reaction of OH to cis-3-hexene have also been performed, at the BHandHLYP/aug-cc-pVDZ level, in order to investigate the reaction mechanism, to clarify the experimental observations and to model the reaction kinetics. Different conformations of the reactants, pre-barrier complexes and saddle points were considered in our calculations. The individual rate coefficients, calculated for each conformer of the reactant, at 298 K, using a microcanonical variational transition state method, are 4.19 × 10⁻¹¹ and 1.23 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The global rate coefficient was estimated from the Boltzmann distribution of the conformers to be 8.10 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which is in agreement with the experimental value. Rate coefficients calculated over the temperature range from 200–500 K are also given. Our results suggest that the complex mechanism, explicitly considering different conformations for the stationary points, must be taken into account for a proper description of the reaction kinetics.