| China is the largest consumer of pesticide in world. Organic nitrogen pesticides and organic phosphorus pesticides are two kinds of most widely used pesticides, which will be released into the atmosphere by spray, volatilization, and wind erosion. Fortunately, they can be removed by reacting with a variety of reactive species, such as OH radicals, NO3 radicals and O3. Among them, OH radicals are believed to be the most important one. Therefore, the study on chemical reactions of typical organic nitrogen and organophosphorus pesticides with OH radicals in the atmosphere is of great significance to further study the reaction kinetics and understand their chemical degradation.The direct dynamics method was employed to study the microscopic mechanism and kinetic properties of reactions for four kinds of carbamate pesticides with OH radicals. The calculated results indicated that the H-abstraction reaction channels of methyl are the major channels, and the reaction activity of methyl becomes higher with the increase of the electronegativity of the group connected with it. The total rate constant value calculated for the reaction of CH3NHC(O)OCH3 with OH radicals at 296 K, 3.88×10-12 cm3molecule-1s-1, is in good agreement with the available experimental value. The three-parameter expressions of the total rate constants for the reaction of four kinds of carbamates pesticides with OH radicals: ka(T) = 4.65 × 10-28 T4.76 exp(2845.48/T);kb(T) = 2.04 × 10-17 T2.07 exp(204.44/T);kc(T) = 2.62 × 10-20 T2.74 exp(1146.08/T);kd(T) = 1.09 × 10-22 T3.59 exp(1144.99/T), were given in the temperature range of 200-1000 K.The density functional theory was adopted to study the degradation mechanism and kinetic properties of carbaryl with OH radicals. The kinetics results show that the H-abstraction reaction on the N—H bond of side chain is the major reaction channel at low temperatures, with the temperature increases, H-abstraction reaction on methyl of side chain becomes more and more competitive. The other channels are the minor channels. The total rate constant at 296 K is of 5.67×10-11 cm3molecule-1s-1, which is consistent with the available experimental value. The three-parameter expression of the total rate constants of carbaryl with OH radicals, k(T) = 8.22×10-20 T 3.05 exp(892.23/T), was given over the temperature range of 200-800 K.The dual-level direct dynamics method was applied to study the degradation process of chlordimeform initiated by OH radicals. The final results indicated that the addition reaction of carbon-nitrogen double bond is the major channel, and the abstraction reaction on the side chain of chlordimeform is the most competitive channel at low temperature. The total rate constant at 296 K, 2.80×10-10 cm3molecule-1s-1, is according with the available experimental value. The three-parameter expression of the total rate constants k(T) = 2.62 × 10-18 T2.71 exp(899.61/T) was given over the temperature range of 200-1000 K.The M06-2x function with the 6-31+G(d,p) basis set were employed to study the reaction process of methamidophos and chloramine phosphorus with OH radicals. By comparing the degradation mechanism of the two kinds of organic phosphorus pesticides, we can find that once the H atom of NH2 group in methamidophos is replaced by the electron withdrawing group of CH(OH)CCl3, the structure of chloramine phosphorus is formed. Apparently, the activity of N—H will become higher making it is easier to be abstracted by OH radicals. The total rate constant at 298 K, 2.59×10-11 cm3molecule-1s-1, is very close to the available experimental value. The three-parameter expressions k1(T) = 1.53 × 10-19 T2.74 exp(-1005.12/T);k2(T) = 1.36 × 10-20 T3.02 exp(-1259.56/T) were given.For all the reaction systems mentioned above, the research results of theoretical calculation are in good agreement with all available experimental data. Therefore, the direct dynamics method and the density functional theory are credible to be employed in the study on reaction mechanism of Organic nitrogen pesticides and organic phosphorus pesticides. They can provide a kind of effective method for the future study of the similar reaction system. In this paper, the values of branching ratios and the expressions of rate constant over larger temperature range also provided some reliable data for further laboratory research. |
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