Heterogeneous Reactions of Pirimiphos-Methyl and Pirimicarb with NO<sub>3</sub> Radicals
2012-11-08T00:00:00Z (GMT) by
Pirimiphos-methyl (PMM) and pirimicarb (PM) are two typical <i>N</i>,<i>N</i>-dialkyl substituted pyrimidine pesticides. The heterogeneous reactions of suspended PMM and PM particles with NO<sub>3</sub> radicals are investigated using an online aerosol time-of-flight mass spectrometer and a real-time atmospheric gas analysis mass spectrometer. Three products for PMM and five products for PM are observed and assigned with the aid of GC/MS. Phosphoric acid 2-diethylamino-6-methyl-4-pyrimidinyl dimethyl ester and 2-(dimethylamino)-5,6-dimethyl-4-hydroxy-pyrimidine are the main reaction products observed for PMM and PM, respectively. The effective rate constants for the reactions of PMM and PM particles with NO<sub>3</sub> radicals are (9.9 ± 0.3) × 10<sup>–12</sup> and (7.5 ± 0.3) × 10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, respectively, obtained using a mixed-phase relative rate method. Geometries and energies of transition states (TS) and intermediates (IM) are obtained by DFT calculation to elucidate the detailed mechanism of the PS group oxidation into the PO group for PMM. The theoretical studies present the reasonable intermediates including the S-oxide and the diradical (IM1<sup>a</sup> and IM2<sup>a</sup>). The mechanism explanation may provide useful information for understanding the degradation mechanism of organothionophosphorus compounds in the environment.
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