Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO-C6H6-O-2) produced from the reaction of OH-benzene adduct with 0, were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2-hydroxycyclohexadienyl peroxy radical H, and 4-hydroxycyclohexadienyl peroxy radical IIIs were calculated at the following theoretical levels, B3LYP/6-31G(d), BRYP/6-311G(dp), and B3LYP/6-311+G(d,p). Both were shown to contain a red-shifted intramolecular hydrogen bond (O-H...O-H bond). According to atoms-in-molecules (AIM) analysis, the intramolecular hydrogen bond in the 2-hydroxycyclohexadienyl peroxy radical IIs is stronger than that one in 4-hydroxycyclohexadienyl peroxy radical R, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red-shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O-H antibonding orbital, while the increased electron density in the O-H antibonding orbital leads to the elongation of the O-H bond and the red shift of the O-H stretching frequency. (c) 2006 Wiley Periodicals, Inc.