Mechanism Study On The Degradation Of Methomyl By Direct Dynamic Methods

Recently, the atmospheric pollution caused by the pesticides has been receivingmore and more recognition. The pesticides can enter the atmosphere through spraydrift during application, post-application volatilization and wind erosion. In ourcountry, the annual output and usage of carbamate pesticides is very huge. Methomylis a kind of widely used, high efficiency, and broad spectrum carbamate pesticides.The toxicity of the degradation products of this kind of pesticide in nature are oftenequal parent compound, the toxicity of some degradation products are even higherthan the parent compound and carbamate pesticides are often detected in theatmosphere. Therefore, the study of the degradation mechanism of methomyl in theatmosphere is very urgent and essential.In this paper, the Gaussian09was performed to calculate and analysis themicroscopic reaction mechanisms of the carbamate pesticides degradation reactioncaused by OH and H radicals in the atmosphere using the MP2method at the6-311+G(d,p) basis set. The transition state models of all the probable reactionchannels of the reaction system was established, the accurate potential energy surfaceinformation of all reaction channels was obtained. The equilibrium geometries,energies, and vibrational frequencies of all the stationary points were calculated, andthe minimum energy path was obtained by intrinsic reaction coordinate (IRC) theory.Potential energy surface informations were further corrected by the MC-QCISDmethod. Polyrate9.7procedures was applied to fit the reaction rate constants andthree-parameter expression about reaction temperature.The results showed that in the four hydrogen abstraction reaction channels ofOH radical degrade methomyl reaction, the energy barrier of the reaction channelthat OH radical abstracts the H atom from the CH3bonding to the S atom (R1a) is thelowest and rate constants for R1a are always dominant over the whole reaction temperature interval. R1a is the main reaction channel and the rest of reactionchannels are secondary. In the four hydrogen abstraction reaction channels of Hradical degrade methomyl reaction, the energy barrier of the reaction channel of Hradical abstracts the H atom from the CH3bonding to the N atom (R2d) is the lowestand rate constants for R1a are always dominant over the whole reaction temperatureinterval. R2d is the main reaction channel and the rest of reaction channels aresecondary.