Hydrogen abstraction from n-butanol by the hydroxyl radical: high level ab initio study of the relative significance of various abstraction channels and the role of weakly bound intermediates.

We have investigated the mechanism of the reaction of H abstraction from n-butanol by the hydroxyl radical (HO*) using high level ab initio methods in conjunction with the correlation consistent basis sets up to quadruple-zeta quality (cc-pVQZ). This reaction is of significance in the atmosphere and combustion. The focus of the study has been on the relative importance of the abstractions from the specific n-butanol sites and on the role of reaction intermediates involved. Our results show that abstractions from the C(alpha) and C(gamma) positions are kinetically most favored and nearly barrierless, with barrier height estimates of 0.10 and 0.47 kcal/mol, respectively, at the CCSD(T)/cc-pVQZ level. We have determined that the indicated barrier height order, C(alpha) < C(gamma) < C(beta) < C(delta) < OH, parallels that for the n-butanol bond dissociation energies established recently. The kinetically and thermodynamically most favored C(alpha) abstraction occurs via a mechanism including the formation of the n-butanol...HO* prereaction complex. The weakly bound postreaction complexes between the product radicals and H(2)O have been identified for all the specific site abstraction reactions, with their calculated CCSD(T) binding energies of up to about 3 kcal/mol after correcting for the basis set superposition error. G3 method has been found to yield consistent results with those obtained from the CCSD(T) calculations for the predicted orders of both the H abstraction barrier heights and their exothermicities.