Theoretical Study of Hydroxyisoprene Alkoxy Radicals and Their Decomposition Pathways
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We report theoretical studies of the alkoxy radicals arising from the OH-initiated reactions of isoprene and their decomposition pathways. Density functional theory (DFT) and ab initio molecular orbital calculations have been employed to determine the structures and energies of the alkoxy radicals as well as the transition states and products of their decomposition reactions. Geometry optimizations of the various species were performed with density functional theory at the B3LYP/6-31G(d,p) level, and the single-point energies were computed using various methods, including second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). The ab initio energetics of the alkoxy radicals along with their transition states and products of decomposition were used to determine the reaction and activation enthalpies of the C-C bond fission of the alkoxy radicals. The results indicate that the calculated energies are very sensitive to the electron correlation effect. For example, at the CCSD(T)/6-311G(d,p) level of theory, decomposition of the β-hydroxyalkoxy radical with OH and O. located at C1 and C2 (respectively) is found to be slightly endothermic (by 2.1 kcal mol-1), with an activation barrier of 8.5 kcal mol-1. Those values are noticeably different from the results obtained using the MP2 and B3LYP methods. Using the obtained activation barriers and the transition state structures, we have calculated the high-pressure limit decomposition rates of the various isomers of the alkoxy radicals. The C-C bond fission is expected to occur readily for the four β-hydroxyalkoxy radicals with the calculated rate constants in the range of 4 × 107 to 6 × 108 s-1, but the rates are much lower for the two δ-hydroxyalkoxy radicals (<3 × 10-2 s-1).
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