Reaction mechanism and kinetics of H and Cl atom abstraction in Dichloromethane with OH radical

Dichloromethane or methylene chloride (DCM, CH2Cl2) is a hazardous toxic pollutant that enters the stratosphere via reaction with OH radicals. The present study examines DCM's complete reaction mechanism with OH radicals via H and Cl atom abstraction using density functional theory (DFT) methods such as B3LYP, M06-2X level of theory with 6-311++G(d,p) and 6-311++G(3df,2p) basis sets. In addition to DFT results, ab initio and composite methods such as MP2, CCSD(T), G3B3, and CBS-QB3 level of theory are utilized to validate the results. The subsequent secondary reaction mechanism of peroxy radicals with HO2 and NO radicals was studied in both singlet and triplet states. H-atom abstraction is the most favorable with a small relative energy barrier of 2.70 kcal/mol. The rate constant calculated by canonical variational transition state theory with the small curvature tunneling method (CVT/SCT) at 298 K is 9.50 x 10-13 cm3 molecule-1 s-1, which agrees well with previous experimental studies, and the atmospheric lifetime of the reaction is 12.18 days at 298 K. Possible new intermediates and products, such as dichloromethyl peroxide, dichloromethyl nitrate, and formyl chloride, have been identified.