Kinetics and oxidation mechanism of pyrene initiated by hydroxyl radical. A theoretical investigation

Oxidation mechanism of Pyrene (Py) by OH center dot radicals is studied in two possible reactions by density functional theory (DFT) at the M06-2X/6-311 + + G(2df,2p) theoretical level. OH-addition reaction in compare with the H-abstraction reaction is a more favored pathway in the gas-phase. Hence, the studies continued for investigating the kinetic aspects of the OH-addition reaction. Furthermore, Rice-Ramsperger-Kassel-Marcus (RRKM) theory as well to calculate the kinetic rate constant for the initial steps and investigate the effects of diversity in pressure and temperature. Pathway 2 (attack onto C-2 carbon atom) and following that forming of the related complex which has a lower energy barrier is the most efficient process. The good agreement with the available experimental data reveals that a two-step reaction scheme prevails. These ratios also show a decrease in the regioselectivity by increasing the temperatures and decreasing the pressures. The Pyrene lifetime in the atmospheric condition with OH center dot is estimated 5.5 days. An atom in Molecules theory (AIM) in critical point and natural bond orbital (NBO) analysis use to describe the nature of covalent interaction, and is as a confirmation of DFT. The NBO analysis shows a decrease in the HOMO-LUMO gaps to form the stable complex, which is parallel with the decreasing of activation energy.