Reaction Mechanism and Kinetics of the Atmospheric Reactions of 2, 3-Dimethylphenol with OH Radical

Abstract

Aromatic compounds such as benzene, toluene and xylene are important constituents of gasoline fuels, solvents and are ambient air
pollutants in urban areas. The atmospheric degradation of xylene leads to the formation of dimethylphenols, which are highly reactive in the atmosphere. The dominant degradation process for dimethylphenols is their reaction with OH radical, which proceeds by H-atom abstraction from the alkyl substituted groups. The resulting alkyl radical further undergo oxidation reaction with O2, leading to he formation of an alkyl peroxy radical. This alkyl peroxy radical further reacts with atmospheric  species to generate new products. In the present work, the possible ring cleaving reaction mechanism for OH initiated reaction of 2,3- dimethylphenol is studied, the different reaction pathways are modeled, and the major product channels are identified by theoretical methods. The  potential energy surface for the reaction system is characterized by using DFT-B3LYP/6-31G(d,p) level of theory. The connectivity of the  transition states with their corresponding reactants and products is confirmed by intrinsic reaction coordinate calculations. The rate constants  of the reaction channels are calculated using canonical variational  transition state theory (CVT) with small curvature tunneling (SCT) corrections over the temperature range of 278-350 K. The formation of 3- hydroxy-2-methyl-2H-pyran-2-carbaldehyde is found to be favorable with a small energy barrier of 7.74 kcal/mol and with a rate constant of 2.22 x 10-11 cm3molecule-1s-1 at 298 K. Thus, the peroxy radical chemistry achieves the conversion of dimethylphenol to unsaturated dicarbonyls and aldehydes, thereby aiding oxidation and combustion processes that either releases large amount of energy or form reactive free radicals.