31P NMR and ESI-MS studies of metal ion-phosphorus pesticide residue complexes

Abstract

31P NMR and ESI-MS structural studies have been carried out for O,O-dimethylphosphorothioate anion (PA–) interacting with Ag+, Hg2+, and Na+ cations. Evidence is presented for the formation of PA–···Ag+ and PA–···Hg2+ metal complexes and for ionic PA–Na+, with 31P chemical shift values of 43, 40, and 65 ppm, respectively. The 31P chemical shifts for PA–···Ag+ and PA–···Hg2+ complexes exhibit an initial sharp rise as the [Ag+]/[PA] and [Hg2+]/[PA] ratio increases from 0 to 1.0 and 0 to 0.5, respectively, and remain almost constant beyond these ratios. These results indicate the formation of 1:1 PA–···Ag+ and 2:1 (PA–)2···Hg2+ complexes. Electrospray ionization mass spectroscopy (ESI-MS) revealed peaks corresponding to [O-P(=S)(OCH3)2 + Ag + H]+ and {2[O-P(=S)(OCH3)2] + Hg + H}+ for PAH interacting with Ag+ and Hg2+, supporting the 1:1 PA–···Ag+ and 2:1 (PA–)2···Hg2+ stoichiometry. The experimental results support the predominance of species with greater P=O double bond and P–S single bond character, relative to P-O/P=S. Our results on 31P NMR chemical shifts have a bearing on the use of 31P chemical shifts as a tool for organophosphorus pesticide identification.