Mechanistic Dichotomy In The Gas-Phase Addition Of No3 To Polycyclic Aromatic Hydrocarbons: Theoretical Study

The gas-phase addition mechanism of the NO3 radical (an important tropospheric nocturnal oxidizing species) to some selected polycyclic aromatic hydrocarbons (PAHs), important pollutants of the troposphere, has been computationally analysed. Purpose of this work is to verify whether the reaction can take place through a mechanism different from the simple radical addition to the aromatic system. This mechanism could consist in an Electron Transfer (ET) from the aromatics to NO3, thus generating an aromatic radical-cation and a nitrate anion at long CO distances. The coulomb attraction should finally bind the two species and generate the radical adduct without any electronic energy barrier. The CASPT2 results show that, while benzene and naphthalene react with NO3 through a plain radical mechanism, anthracene reacts by a mechanism with a partial ET character, and pentacene reacts with a sort of inner-sphere ET pathway. These results concur to explain the high reactivity of NO3 with larger PAHs whose ionization energy is below 7 eV and could be important in studies of environmental PAH oxidative degradation.