Mechanism theoretical study on OH-initiated atmospheric oxidation degradation of dimethoate

Quantum chemical calculations were performed to investigate the mechanism of OH radical-initiated atmospheric oxidative degradation of dimethoate (DMA). Geometrical parameters and vibrational frequencies were calculated at the MPWB1K/6-31 + G (d,p) level and the energies of reactants, transition states, intermediates, and products were calculated at the MPWB1K/6-311 + G (3df,2p) level. Rate constants of the key elementary steps with positive potential barriers were obtained by utilizing the canonical variational transition-state (CVT) theory in combination with the small curvature tunneling (SCT) correction over a temperature range of 273-333 K. Both H abstraction from the CH3 group (in NHCH3 group) and the generation of more toxic omethoate (OMA) via OH-adduct are barrierless processes, which reveals that these pathways are thermodynamically favorable and OMA could be one of the major intermediates in DMA degradation process in the atmosphere. The calculated rate constants provide new insights into the lifetime of dimethoate in the atmosphere.