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The compared kinetics of the reactions of unsaturated alcohols and alkenes with OH radicals is a topic of great interest from both the theoretical chemistry and the atmospheric chemistry points of view. The enhanced reactivity of an unsaturated alcohol, with respect to its alkene analogue, toward OH radicals has been previously demonstrated, at 298 K, by experimental and theoretical research. In this work, a new comparative investigation of such reactions is performed for 3-buten-1-ol and 1-butene. The model assumes that the overall kinetics is governed by the first OH addition steps of the mechanism. Calculations have been performed at the DFT level, employing the BHandHLYP functional and the cc-pVDZ and aug-cc-pVDZ basis sets, and the rate coefficients have been determined on the basis of the microcanonical variational transition state theory. The rate coefficients obtained for the OH reactions with 3-buten-1-ol (kOH(31BO)) and 1-butene (kOH(1B)) at 298.15 K are lower than the experimental rate coefficient available in the literature, showing deviations of 18% and 25%, respectively. Negative temperature dependence is verified for these rate coefficients. The kOH(31BO)/kOH(1B) ratios have also been investigated as a function of the temperature, suggesting that at room temperature the unsaturated alcohol reacts with the OH radicals faster than 1-butene, by a factor of 1.2, but at higher temperatures (400-500 K), the alkene should react faster, and that the stabilization of prebarrier complexes and saddle points due to hydrogen bonds is no longer an important factor to govern the reactivity of the unsaturated alcohol toward OH radicals, with respect to the alkene analogue.

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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.