Theoretical Study On The Degradation Mechanism Of Pyrethroids

As a kind of environment friendly pesticide, pyrethroid is considered to be a good substitute for highly toxic organophosphorus pesticides. At the present time it's been widely used in agriculture practice, while at the same time it has drawn more and more attention about its safety. The research on its transport and transform mechanism is of great realistic significances. Since its degradation process in water is directly related to human health, it's therefore especially important to have a insight view to the degradation mechanism of pyrethroids in water.In this thesis, we first studied the degradation reaction mechanism of fenvalerate in water using density functional theory at the B3LYP/6-31G* level. The structure of transition state was searched and optimized, on the basis of optimizing the geometric structure of reactant and products. The transition state and reaction pathway were confirmed by the vibration analysis and inner reaction coordinate (IRC) tracking method. The natural population analysis (NPA) and the natural bond orbital (NBO) were also used to help checking the details of reaction process, such as the breaking of old bonds and the forming of new bonds. The calculated activation energy of the reaction is 133.4kJ/mol, and the exothermic energy is 11.0 kJ/mol.From the geometric parameters of the reactant and the transition state, it's shown that, in the transition state, the bond of hydroxyl free radical is drawn longer, while the double and single bonds of fenvalerate linked with C(1) are getting longer with smaller bond order and weaker bond intensity. Meanwhile the double bond of C(1)=O(2) and the phenyls twist a lot. Only one false vibration frequency, located at -427cm-1 and correspond to the vibration of bond C(1)-O(31), was found in the calculated IR spectrum of the transition state. An Einstein shift was noticed for the telescopic vibration frequency of bond C(1)-O(2) and C(1)-O(3), accompanied by the weakening of vibration intensity, indicating that the bond-forming between the oxygen atom of hydroxyl free radical and the carbon atom of carbonyl group of fenvalerate weakens these bonds.From the natural population analysis (NPA), it is seen that, after the attack of the hydroxyl free radical to carbon atom of carbonyl group of fenvalerate, the electronegativity of both atom C(1) and O(2) of the carbonyl group become higher. This is mainly caused by the electrons transfer from the hydroxyl free radical to the carbonyl group, which is again an indication of their interaction.The natural bond orbital analysis (NBO) showed that, the stabilization energy E between the bonding orbital of orphaned doublet of atom O(31) in hydroxyl free radical and the antibonding orbital of orphaned doublet of atom C(1) is quit big, suggesting that there is significant interaction between the hydroxyl free radical and the carbonyl group. Again the bond-forming between them in transition state was confirmed. The attack of the hydroxyl free radical induces changes of the bonding of carbonyl group and other vicinal bonds. It is also revealed that there are fairly strong interactions among the atomic orbitals of the three phenyl of fenvalerate.In summary, all the results and details analysis testified our judgment to the reaction mechanism. That is: upon the attack of hydroxyl free radical, the bonding of the carbonyl group C(1)=O(2) become weaker, resulting in a weak bond between atom C(1) and atom O(31) come into being and the formation of the transition state form. In the following step, the bonding between C(1) and O(3) in the transition state breaks, with the formation of a new bond C(1)-O(31). A SN2 substitute reaction thus finishes with the generation of an alcohol and an acid product.For comparison purpose, three other representative pyrethroids—cypermethrin, methothrin and ethofenprox—were chosen to conduct similar calculations. Differences among the structure of reactant, the transition state and the reaction process were demonstrated.