Ab Initio Molecular Orbital Calculations of C₆H₅O₂ Isomers

The structures and energetics of 14 geometric isomers of C₆H₅0₂, including phenyl peroxy (C₆H₅OO), hydroperoxy phenyl (C₆H₄OOH), seven-membered ring C₆(O)H₅O isomer, three isomers of hydroxyl phenoxy [C₆H₄O-(OH)], seven isomers of OC₆H₅O (including the three with epoxy structures), and a stable dual-ring C₆H₅-O₂ π-complex have been calculated by ab initio molecular orbital methods. Geometries have been optimized at the UHF/6-31G^* level, and their relative energies with respect to C₆H₅ + O₂ have been refined by the spin-projected UMP3(PUMP3) method with UHF/6-31G* zero-point energy corrections. All C₆H₅O₂ isomers identified except w-OC₆H₅O, which is not a local minimum, are more stable than the C₆H₅ + 0₂ reactants. The most stable isomers are the three C₆H₄O-(OH) radicals, which are 105-110 kcal/mol below C₆H₅ + O₂. They are followed by the seven-membered ring C6-(O)H₅O, the m-epoxy isomer, other C₆H₅O radicals, and the initial adduct C₆H₅OO, which is 41 kcal/mol lower than C₆H₅ + O₂. The ir-complex, formed by the association of C₆H₅ with O₂ at the center of the O═O bond, is 27 kcal/mol below the reactants. The C₆H₄OOH radical, which may be formed by intramolecular H-abstraction, is more stable than C₆H₅ + O₂ by 9 kcal/mol. The results suggest that several mechanisms may be involved in the C₆H₅ + O₂ reaction. Vibrational frequencies determined for each isomer at the UHF/6-31G* level of theory have been discussed.