Theoretical study of the intramolecular hydrogen bonds of aromatic peroxy radicals from OH-toluene reactions

Three methyl-hydroxycyclohexadienyl peroxy radicals with intramolecular H-bonds were studied at B3LYP/6-31G(d), B3LYP/6-311+G(d,p) and B3LYP/6-311++G(2d,2p) levels, respectively. 2-hydroxy-1-methyl-cyclohexadienyl peroxy radical (II), 2-hydroxy-3-methyl-cyclohexadienyl peroxy radical (III), and 4-hydroxy-3-methyl-cyclohexadienyl peroxy radical (IV) exhibit red-shifted O-H⋯O H-bond. From the AIM analysis, it becomes evident that 2-hydroxy-3-methyl-cyclohexadienyl peroxy radical (III) intramolecular O-H⋯O H-bond is the strongest. This represents that 2-hydroxy-3-methyl-cyclohexadienyl peroxy radical (III) is the most stable isomer. It is clear that hydrogen bonding plays a role in stabilizing the peroxy radicals: the hydrogen bond lengths are 1.9804, 1.9566, and 2.4457 Å for (II), (III), and (IV), respectively, correlating with their relative stability. From the NBO analysis, it becomes evident that three red-shifted intramolecular H-bonds can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization. The hyperconjugation is slightly dominant and overshadows the rehybridization in the O-H⋯O H-bond.