On the role of s-cis conformers in the reaction of dienes with OH radicals

The reactions of OH radicals with s-cis and s-trans-butadiene and s-cis-isoprene have been modeled by ab initio Molecular Orbital Theory. Density Functional Theory (BHandHLYP) calculations have been performed for both butadiene and isoprene, and Moller-Plesset Perturbation Theory to the Second-order (MP2) has also been used for s-cis-isoprene in order to compare with previous work. Pre-reactive complexes are. identified in all cases, with the OH radical placed over either one of the double bonds at a distance of about 2 Angstrom and the H atom pointing towards the C-C bond. The geometries of the transition states corresponding to OH addition at all positions have been fully optimized. The calculated apparent activation energies are negative for addition at the terminal carbon atoms and in excellent agreement with the experimental measurements. The possible role of direct additions at the internal carbon atoms in the formation of furan-like compounds is discussed. Energy barriers for the s-cis conformers are found to be smaller than those for the s-trans conformers, especially for addition at the internal carbons, suggesting that the s-cis conformers could play a role in the tropospheric oxidation of dienes. Calculated overall rate constants are in good agreement with experimental values. Partial rate coefficients corresponding to the different channels are reported. The temperature dependence is studied in the 290-500 K range and two-parameter equations are reported for each rate coefficient. The calculated partial rate coefficients of addition to internal carbon atoms are not large enough to account for the observed yield of 3-methylfuran.