The Reaction Mechanism And Kinetics For The Reaction Of OH Radicals With Atmospheric Metolachlor

Metolachlor[2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide],has been used as a chloroacetanilide herbicide to control annual grass weeds and broadleaf weeds in corn,cotton,peanuts,soybeans and beans.The widely use of metolachlor has led to contamination of atmosphere,rainwater,groundwater,soil and non-target organisms.Metolachlor is found to be related with decreased cell proliferation,growth and reproductive ability of non-target organisms.In the atmosphere,metolachlor exist mainly in the gas phase and can undergo atmospheric reactions with ubiquitous OH,O3,NO3 and so on.During their degradation processes,more toxic secondary pollutants may be formed.Understanding the atmospheric degradation processes of metolachlor is important for removing this substance from the atmosphere and assessing its potential risks to non-targets.In this paper,aRS-metolachlor and aSR-metolachlor has been used as models to investigate the reaction of OH radicals with atmospheric metolachlor.The reaction mechanism was obtained at the MPWB1K/6-311+g(3df,2p)//MPWB1K/6-31+g(d,p)level of theory and the rate constants were deduced over the temperature range of 180-370 K using canonical variational transition state(CVT)theory with the small curvature tunneling(SCT)method.The atmospheric lifetime of aRS-metolachlor and aSR-metolachlor determined by OH radicals are about 3.97 hours and 3.41 hours,which indicates that it can be degradaded in the gas phase easily and doesn't have the potential for long-range transport.