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) cm(3) 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) cm(3) molecule(-1) s(-1). The global rate coefficient was estimated from the Boltzmann distribution of the conformers to be 8.10 × 10(-11) cm(3) 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.

Rate Coefficient for the Reaction of Cl Atoms with cis-3-Hexene at 296 ± 2 K

“…As previously discussed, 3 these uncertainties have been calculated by assuming both the standard error of the slopes of the logarithm concentration curves and the reported errors on the reference rate coefficients. 6,13 Errors due to sample handling and chromatographic method were introduced in the standard error of the slope.…”

“…A comparison of the room temperature rate coefficients determined for the reactions of cis-3-hexene with Cl atoms and OH radicals 3 shows that the former is 6.6 times higher. In order to (i) evaluate the rate coefficient for the reaction of Cl atoms with cis-3-hexene determined in this work, (ii) explain the higher reactivity towards Cl atoms and (iii) to infer about the reaction mechanism, the reactivity of a series of alkenes was compared and the contributions of hydrogen abstraction and electrophilic addition channels were evaluated.…”

“…The effective lifetimes of cis-3-hexene with respect to reaction with each of the oxidants were calculated using the relationship expressed as: (9) with X = Cl atoms, OH and NO 3 30 and considering average global concentrations of 1 × 10 4 atoms cm -3 of chlorine. 31 From the rate coefficients available in the literature [3][4][5] and the experimental rate coefficient obtained in this work, the lifetimes were calculated and ranged from 1.27 h to 2.8 days. These calculations do not take into account local atmospheric conditions and seasonal variations which are capable of changing these oxidant concentrations.…”

“…1,2 In the troposphere, cis-3-hexene reacts with hydroxyl radicals, ozone and nitrate radicals. Barbosa et al 3 have studied the kinetic for the reaction of cis-3-hexene with hydroxyl radicals using the relative rate method and the experimental mean second-order rate coefficient value was determined as (6.27 ± 0.66) × 10 -11 cm 3 molecule -1 s -1 . The kinetics of the reaction with ozone and nitrate radicals have been investigated at room temperature using the relative method and the rate coefficients values (in cm 3 molecule -1 s -1 ) have been determined as (1.44 ± 0.17) × 10 -16 and (4.37 ± 0.49) × 10 -13 , respectively.…”

“…This methodology has been carried out at this laboratory for the study of other reactions. 3,[7][8][9] Briefly, the reactant and the reference compound were introduced into the chamber using nitrogen or ultrapure air. Cl atoms were generated by the trichloroacetyl chloride photolysis using three germicide lamps (Philips 30 W), with a λ maximum around 254 nm, and the time of photolysis varied from 20 seconds to 1 minute.…”