Speciation of VX in Aqueous Solutions

P-31 and C-13 NMR studies of the toxic chemical warfare agent VX [O-ethyl S-(2-N,N'-(dlisopropylamino)ethyl) methylphosphonothioate] have been used to understand the reactivity in a wide variety of matrices. The presence of protonated VX has been invoked in many mechanisms of VX degradation and decontamination. A change of speciation of VX has been justified by a relatively large shift in the P-31 NMR chemical shift from about 55 ppm (in neat material or organic solvents) to >60 ppm (in aqueous solutions). This study shows that in acetonitrile/water mixtures and in aqueous solutions at a range of pH values, the P-31 NMR resonance was sensitive to the percentage water in the mixture, but was not sensitive to the pH of the solution. This result indicates that the P-31 chemical shift to >60 ppm is not associated with protonated VX, which is only present at pH < 8. However, the C-13 NMR resonances did respond to changes in the pH of the solution, indicating that these shifts are associated with protonation of the VX in acidic media.