MO study of the benzylperoxyl radical, an intermediate in atmospheric ozone formation

Unrestricted Hartree-Fock calculations have been carried out to study the benzylperoxyl radical, an intermediate atmospheric contaminant. For this purpose, quantum mechanical MP2//UHF ab initio calculations were performed by using 6-31G∗∗ and 6-31 + G ∗∗ basis sets. Because no previous experimental nor theoretical data were available for benzylperoxyl, the results were compared with those of the smaller unsaturated peroxyl radicals, such as allylperoxyl. It is found that the calculated absolute minimal energy corresponds to a trans conformation in which the OO bond is sticking away from the benzene ring. Benzylperoxyl is found also to exhibit an unusually OO small bond order, which reflects the weakness of the bond, i.e. its ability to react with NO forming NO2 and subsequently ozone. A peculiar MO diagram is obtained, with the unpaired MO deeply located in the “doubly” occupied MO space. This effect is also observed in allylperoxyl. Structural parameters, charges, spin densities and dipole moments are also reported.