Theoretical study of hydroxyisoprene alkoxy radicals and their decomposition pathways

We report theoretical studies of the alkoxy radicals arising from the OK-in itiated reactions of isoprene and their decomposition pathways. Density fun ctional theory (DFT) and ab initio molecular orbital calculations have been employed to determine the structures and energies of the alkoxy radicals a s well as the transition states and products of their decomposition reactio ns. Geometry optimizations of the various species were performed with densi ty functional theory at the B3LYP/6-31G(d,p) level, and the single-point en ergies were computed using various methods, including second-order Moller-P iesset perturbation theory (MP2) and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple e xcitations (CCSD(T)). The ab initio energetics of the alkoxy radicals along with their transition states and products of decomposition were used to de termine the reaction and activation enthalpies of the C-C bond fission of t he alkoxy radicals. The results indicate that the calculated energies an ve ry sensitive to the electron correlation effect.. For example, at the CCSD( T)/6-311G(d,p) level of theory, decomposition of the beta -hydroxyalkoxy ra dical with OK and O-. located at C1 and C2 (respectively) is found to be sl ightly 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 obtai ned 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. Th e C-C bond fission is expected to occur readily for the four beta -hydroxya lkoxy radicals with the calculated rate constants in the range of 4 x 10(7) to 6 x 10(8) s(-1), but the rates are much lower for the two delta -hydrox yalkoxy radicals (<3 x 10(-2) s(-1)).