Theoretical Investigations On Degredation Mechanisms Of Three Organophosphorus Pesticides

Organophosphorus pesticides are compounds which include C-P,C-S,C-O and P=O chemical bonds.Because of highly effective antimicrobial properties,organophosphorus pesticides are widely used in agriculture,therefore,their degradations become focus of environmental protection.The main loss way of these organophosphorus pesticides in atmosphere is the reaction with OH radicals.Employing quantum chemical?density functional theory?method,we focus on three organophosphorus pesticides,i.e.,methyl-N,N,N',N'-tetramethyl phosphorodiamidate?MTMPDA?,ethyl-N,N,N',N'-tetramethylphosphorodiamidate?ETMPDA?and O,S-Dimethyl Methylphosphonothioate?OSDMMP?,explore their reaction mechanisms with OH radicals in the atmosphere.Moreover,we study the subsequent reactions of products with strong oxidants,such as O₂ and NO,and to confirm the toxicity of the final degradation products in environment.The results were shown as follows: 1.At two levels of theory,i.e.,?B97XD/6-311++G?3df,2pd?//?B97XD/6-31+G?d,p?and M062X/6-311++G?3df,2pd?//M062X/6-31+G?d,p?,the optimized geometricalparameters of all involved stationary points are close to each other,and the same tosingle point energy.2.For MTMPDA+OH reaction,two H-abstraction channels involved,i.e.,from –CH₃and –NH₂ group.The latter one is predominant with lower barrier surmounted.However,both channels are feasible in atmosphere.The subsequent reactions ofproduct?CH₃?3CH₂N₂P?=O?OCH₃ with O₂ and NO have no obvious effect ontoxicity of MTMPDA finally.While the subsequent reactions of?CH₃?4N₂P?=O?OCH₂ will reduce toxicity of the initial reactant MTMPDA.3.For ETMPDA+OH reaction,three channels take place to form different products,and the barrier of OH abstract H from –CH₂-is much lower than the other channels,thus?CH₃?4N₂P?=O?OCHCH₃ is dominant.The secondary reaction of?CH₃?3CH₂N₂P?=O?OCH₂CH₃ will make minor contribution to the toxicity ofETMPDA,while?CH₃?4N₂P?=O?OCH₂CH₂ and?CH₃?4N₂P?=O?OCHCH₃ reactionswith NO and O₂ will really reduce toxicity of ETMPDA.4.For OSDMMP + OH reaction,three H-abstraction channels exist,as well,to giveout CH₃SP?=O??CH₃?OCH₂,CH₂SP?=O??CH₃?OCH₃ and CH₃SP?=O??CH₂?OCH₃.The former two products surpass lower barriers than the third one,and thus they aremain products.Moreover,CH₃SP?=O??CH₃?OCH₂ and CH₂SP?=O??CH₃?OCH₃occur subsequent reaction with O₂ and NO in atmosphere will reduce toxicity ofOSDMMP,while reaction of these oxidants with CH₃SP?=O??CH₂?OCH₃ will playnull significant effect on OSDMMP.