Theoretical modeling of surface functionalization of coronene by oxidation reactions with OH

In this work, we study the interaction between OH radicals with black carbon BC modeled by a coronene molecule by means of quantum chemistry calculations MOPAC, DeMon-nano, DeMon2K packages with the purpose of understand the aging process of this material. Results show that strong OH radical adsorption is preferred on border sites than on center ones, independently of the theoretical method employed. Results of potential energy curves for OH adsorption show that a chemisorption state may occur with small barrier slower than 3.2 kcal/mol. At the center site a physisorbed state may take place. Once OH chemisorption happens, a dipole moment is created and the hydrophobic coronene surface is transformed to hydrophilic. For that reason water molecules are stabilized in the hydroxylated coronene surface by O^{…}H interactions. Chemisorption of many OH radicals, especially at edge sites, produces coronene hydroxylation that after subsequent OH attacks would lead to H abstractions directly from coronene and from chemisorbed OH. The last feature yield a C-C bond breaking at the coronene edge and the formation of hydroxyl and carboxylic groups. Both reactions were confirmed with DFT calculations using coordinates obtained by PM6. Small reaction barriers in both processes were found. From these preliminary results, it is concluded that a detailed modeling of OH interactions with a coronene molecule may shed some light in the formation of volatile oxygenate compounds and therefore the BC aging in the atmosphere.