Heterogeneous Reactions of Pirimiphos-Methyl and Pirimicarb with NO₃ Radicals

Pirimiphos-methyl (PMM) and pirimicarb (PM) are two typical N,N-dialkyl substituted pyrimidine pesticides. The heterogeneous reactions of suspended PMM and PM particles with NO₃ 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 NO₃ radicals are (9.9 ± 0.3) × 10^–12 and (7.5 ± 0.3) × 10^–13 cm³ molecule^–1 s^–1, 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 PS group oxidation into the PO group for PMM. The theoretical studies present the reasonable intermediates including the S-oxide and the diradical (IM1^a and IM2^a). The mechanism explanation may provide useful information for understanding the degradation mechanism of organothionophosphorus compounds in the environment.