Mechanism of OH-Initiated Atmospheric Photooxidation of Dichlorvos:  A Quantum Mechanical Study

Abstract

Dichlorvos (2,2-dichlorovinyl phosphate, DDVP) is a widely used organophosphorus insecticide. DDVP may be released into the atmosphere, where it may be transported for long distances and undergo chemical transformations. The mechanisms of the atmospheric reactions of DDVP have not been fully understood because of the short lifetime of its oxidized radical intermediates and the extreme difficulty in the detection of these species experimentally. In this paper, we carried out molecular orbital theory calculations for the OH-initiated atmospheric photooxidation of DDVP. The profile of the potential energy surface was constructed, and the possible channels involved in the reaction are discussed. Several energetically favorable reaction pathways are revealed for the first time. The calculated results were compared with the available experimental observations. Four product pathways are energetically feasible for DDVP degradation initiated by OH radicals in the atmosphere and are consistent with the experimentally observed products CCl2O and CO, but the additional products CCl2CHO, (CH3O)2P(O)OH, HO2, and a closed-shell organophosphorus compound denoted P10 are also predicted.

Atmospheric degradation mechanisms of a simulant organophosphorus pesticide isopropyl methyl methylphosphonate: A theoretical consideration

“…[1,2,9,[11][12][13][14][15][16][17] Previous experimental and theoretical studies on OPs containing AOCH 3 or AOCH 2 A reported that reactions of these OPs with O 2 /NO result in the formation of an alkoxy radical ([P(O)OCH(OÁ)A). [12,15,[18][19][20][21][22] These results are in agreement with a subsequent rearrangement of the alkoxy radical occurring through a five-member transition state with the formation of [P(O)OH and a molecular carbonyl radical. In a recent experimental study, [23] the rate constant for atmospheric reaction of OH radicals and a simulant OP containing the AOCH\ and AOCH 3 groups isopropyl methyl methylphosphonate (IMMP, (CH 3 ) 2 CHO-P(O)(CH 3 )OCH 3 ) was determined to be 6.72 Â 10 À11 cm 3 mol À1 s…”

“…This reaction surmounts a relatively low energy barrier of 9.92 kcal/mol in comparison to the Hrearrangement paths. Further removal of CH 3 radicals with atmospheric O 2 would lead to the formation of CO. [21,23] In P8, (Figure 10). In the dissociation process, (CH 3 ) 2 CHOP(O)(CH 3 )OH (P9) which was reported in a previous experimental study, [23] can be formed via a fivecentered transition state (TS15) by in this study.…”

“…This result is consistent with a previous theoretical study that [P(O)AOCH 2 O radical passes through a five-member transition state to form [P(O)OH and a molecular carbonyl radical. [21] Cleavage of the C4AO3 bond and H10 atom migration onto O1 requires an energy barrier of around 11.46 kcal/mol and consequent the formation of P9 is exothermic by 4.71 kcal/mol. In P9, the PAO3 distance is shortened to 1.50 Å from 1.64 Å in IM10, while the length of the newly formed H10AO1 bond is reduced to 0.99 Å .…”

Kinetics and mechanism for degradation of dichlorvos by permanganate in drinking water treatment

“…Compared to other pesticides, dichlorvos is generally preferred because of its cost-effectiveness and wide range of bioactivity. Its annual worldwide sales in 2003 were about 40 million U.S. dollars (Zhang et al, 2007). In China, an estimated demand was over 40,000 tons in 2007, and its usage is expected to increase since five highly toxic organophosphate insecticides (e.g., parathion) have been recently banned.…”

“…The abundant use of dichlorvos and two other pesticides, naled and trichlorfon which can be degraded to dichlorvos (U.S. Environmental Protection Agency, 2006;Zhang et al, 2007), indicates that the occurrence of dichlorvos in water resources may be expected. Gao et al (2009) have recently reported that dichlorvos was the most frequently detected organophosphate pesticide in the surface water samples collected from 600 sites around China.…”