DFT and MP2 molecular orbital determination of OH-toluene-O-2 isomeric structures in the atmospheric oxidation of toluene

We have performed an exhaustive theoretical study, using a density function al theory (DFT) and nb initio techniques, of the possible isomers of the OH -toluene-O-2 radical. DFT calculations of the all electron type using the h ybrid B3LYP approach and 6-31G* orbital basis set were employed. Ln additio n to the well-established ortho position, addition of OH at C-1 on the benz ene ring of toluene was also considered for the initial methylhydroxycycloh exadienyl adduct. Ln all, 28 different intermediate structures of the OH-to luene-O-2 system, consisting of peroxyl radicals, bicyclic structures, and epoxides, have been explored through fully optimized electronic structure c alculations. Starting from the 1,3-O-2-methylorthohydroxycyclohexadienyl ra dical, or ortho-OH adduct, several peroxyl radicals are found to have low-l ying structures contained within a small energy range (about 1 kcal/mol). O nly two bicyclic structures are stable with respect to the methylhydroxycyc lohexadienyl radical plus O-2, one of them being clearly favored. The four possible epoxy structures are all found to lie more than 15 kcal/mol lower than any of their peroxyl and bicyclic isomers. The preference, first noted by Bartolotti and Edney, for structures in which the OH group lies on the same side of the ring as the O-2 group, is obeyed in all cases. If the 1-CH 3, 1-OH cyclohexadienyl radical (or C-1-OH adduct) is used as the initial a dduct, three peroxyl radicals are expected to be formed, while two bicyclic structure and three epoxides need to be considered. These structures are f ound to be, in general, less stable than the ones arising from the ortho ad duct. However, the 4-0, 2,3-epoxy l,l-methylhydroxycyclohexadienyl radical is found to be the most stable of all the isomers considered, and this, by more than 3 kcal/mol. in this work, most structures were also calculated wi th the MP2 method with a 6-31G* basis set. The geometries obtained with the two methods are similar. Contrary to the B3LYP method, MP2 always yields a n extra stability to structures in which the C-1 carbon atom has sp(3) hybr idization. (C) 2000 John Wiley & Sons,Inc.