Theoretical study on the degradation mechanism of methamidophos and chloramine phosphorus with OH radicals

Theoretical investigation on the gas‐phase degradation reaction mechanism of methamidophos (MAP) and chloramine phosphorus (CHP) with OH radicals is performed. The equilibrium geometries and the harmonic vibration frequencies of the stationary points are obtained at M06‐2x/6‐31+G(d,p) level, and the higher‐level energetic information is further refined at M06‐2x/6–311++G(3df,2p) level. The rate constants for the 14 reaction channels are calculated by the improved canonical variational transition state theory with small‐curvature tunneling correction over the temperature range 200–2000 K. The three‐parameter expressions of k1(T) = 1.53 × 10−19T2.74exp(−1005.12/T), k2(T) = 1.36 × 10−20T3.02exp(−1259.56/T) are given. The total rate constants of all reaction channels of MAP with OH radicals are in good agreement with the available experimental data. Our results indicate that the H‐ion reactions on methyl are the major channels for the reaction of MAP and CHP with OH radicals. © 2015 Wiley Periodicals, Inc. Methamidophos and chloramine phosphorus are organophosphate pesticides in common use in agriculture. They can be eliminated in the atmosphere by degradation reactions with OH and NO3 radicals and ozone. First‐principle modeling can provide a fundamental insight into the reaction mechanisms of degradation of these species by OH radical. Simulation results suggest that the H‐ion steps on methyl are the major channels for these reactions.