Mechanism of atmospheric photooxidation of aromatics: A theoretical study

The mechanisms of atmospheric photooxidation of aromatic compounds are of seminal importance in the chemistry of the urban and regional atmosphere. It has been difficult to experimentally account for the full spectrum of oxidation products in laboratory studies. In an effort to fully elucidate the atmospheric reaction pathways for the aromatic-OH reaction, we have conducted theoretical calculations on aromatic intermediates. Energies have been determined for these intermediates by using semiempirical UHF/PM3 geometry optimizations combined with ab initio calculations using density functional theory (DFT). A hybrid DFT model, the Becke3 parameter function with the nonlocal correlation function of Lee, Yang, and Parr, was used in conjunction with the 6-31G(d,p) basis set to study the intermediate structures. Full mechanisms for the OH-initiated photooxidation of toluene, m-xylene, p-xylene, 1,2,4-trimethylbenzene, and m-ethyltoluene are developed. The lowest energy intermediates have been determined, and predicted products from these structures are compared to available experimental product data. These studies serve to refine proposed mechanisms currently available for toluene, m-xylene, and p-xylene, while providing new information on the 1,2,4-trimethylbenzene and m-ethyltoluene reaction pathways.