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运用密度泛函(DFT)理论,研究了农药甲胺磷分子在碱性条件下,水解微观反应机理和速率常数。在B3LYP/6-31+G(d,p)水平上分别优化了水溶液相反应途径上各反应态的几何结构,溶剂化模型采用连续极化连续模型(CPCM)。并用频率分析方法和内禀反应坐标(IRC)计算,分别确认了过渡态的结构和反应途径。计算结果表明,反应决速步骤为第一步氢氧根离子进攻反应物,对3个不同离去基团(甲氧基TS2、甲硫基TS1和氨基TS5)活化自由能分别为15.64 kcal/mol、17.19 kcal/mol和25.80 kcal/mol。采用经Wigner校正的Eyring过渡态理论,计算了(100～1200)K温度范围内的各通道反应速率方程分别为,InK(C-O)=11.19524-2.37134×1000/T;Ink(C-S)=11.68184-3.28612×1000/T;Ink(C-N)=12.03234-7.73338×1000/T,3个方程的相关系数分别为R=-0.99809;-0.99891;-0.99981。计算结果显示甲胺磷在碱性条件下不稳定,能水解,并首先解离的是甲氧基和甲硫基,反应具有较大的自发反应趋势,氨基在碱性条件下不易离去。研究结果为解析有机磷杀虫剂在不同条件下的水解反应机制提供理论依据。
The density functional theory (DFT) was used to study the microscopic reaction mechanism and rate constant of the pesticide methamidophos under alkaline conditions. At B3LYP / 6-31 + G (d, p) level, the geometry of each reaction state in the aqueous solution phase reaction was optimized, and the continuous polarization continuous model (CPCM) was used as the solvation model. The frequency analysis method and intrinsic reaction coordinate (IRC) calculations were used to confirm the structure and reaction pathway of the transition state. The calculation results show that the rate-determining step is the first-step hydroxyl ion reaction reagent, and the free activation energies for three different leaving groups (methoxy TS2, methylthio TS1 and amino TS5) are respectively 15.64 kcal / mol, 17.19 kcal / mol and 25.80 kcal / mol. The reaction rate equations of each channel in the temperature range of (100 ~ 1200) K were calculated using the Eyring transition state theory calibrated by Wigner, respectively. InK (CO) = 11.19524-2.37134 × 1000 / T and Ink (CS) = 11.68184- 3.28612 × 1000 / T; Ink (CN) = 12.03234-7.73338 × 1000 / T, the correlation coefficients of the three equations are R = -0.99809, -0.99891 and -0.99981, respectively. The calculated results show that methamidophos is unstable and hydrolyzed under alkaline conditions, and the first dissociation is methoxy and methylthio. The reaction has a tendency of spontaneous reaction, and the amino group is not easy to leave under alkaline conditions. The results provide a theoretical basis for analyzing the mechanism of hydrolysis reaction of organophosphorus insecticides under different conditions.