Atmospheric degradation mechanisms of a simulant organophosphorus pesticide isopropyl methyl methylphosphonate: A theoretical consideration

摘要

Calculations using density functional theory were performed to explore the mechanisms for atmospheric degradation of isopropyl methyl methylphosphonate (IMMP). The potential energy surface profiles for OH-initiated reaction of IMMP were constructed, and all possible degradation channels were considered. Rate constants were further calculated using transition state theory. It was established from these calculations that H-abstractions from alkyl groups have much lower energy barriers than substitutions of alkoxyl groups, and four possible H-abstraction channels are competitive. Investigations into the secondary reactions under the presence of O_2/NO were also performed. It is shown that O_2 addition, reaction of peroxide radicals with NO to form RO radicals, and removal of ·RO are the major degradation pathways for alkyl radicals. Four selected products, CH_3OP(O)(CH _3)OC(O)CH_3, CH_3OP(O)(O)CH_3, (CH _3)_2CHOP(O)(CH_3)OH, and (CH_3) _2CHOP(O)(CH_3)OCH=O, are predicted to be the major products in this study.