Scavenging of OH and OOH radicals by polyradicals of small polycyclic aromatic hydrocarbons.

The hydroxyl and peroxyl radicals, particularly the former, occur abundantly and damage almost all types of materials. Polycyclic aromatic hydrocarbons (PAHs) and their polyradicals (all hydrogens removed) have been considered as models for graphene in some recent studies. Geometries of different adducts of polyradicals of two small PAHs having four and nine benzene rings with an OH or OOH radical each were optimized employing unrestricted density functional theory and two different density functionals. The ground states of all the adducts involving the PAHs had doublet spin multiplicity while those involving the polyradicals had doublet, quartet, sextet, or octet spin multiplicity that was decided on the basis of calculated minimum total energies for optimized geometries. Binding energies of the adducts of an OH or OOH radical at the different sites of the polyradicals of PAHs showed that the OH radical would bind with these systems much more strongly than the OOH radical while both the radicals would bind much more strongly with the polyradicals than with the PAHs. Furthermore, both the OH and OOH radicals are found to bind at the edges of the polyradicals much more strongly than at their interior sites. It is shown that polyradicals can serve as efficient scavengers of OH and OOH radicals and therefore, these materials can be used to protect both biological and non-biological systems from damage due to reactions with these radicals.