Force Fields for Molecular Modeling of Sarin and its Simulants: DMMP and DIMP

Even three decades after signing the Chemical Weapons Convention, organophosphorus chemical warfare agents (CWAs), such as sarin, remain a threat. The development of novel methods for the detection of CWAs, protection from CWAs, and CWA decontamination motivates research on their physicochemical properties. Due to the extreme toxicity of sarin, most of the experimental studies are carried out using less toxic simulant compounds. In addition to experimental studies of sarin simulants, both sarin and simulants can be studied using in silico experiments-molecular simulations. The results of classical molecular modeling of the compounds and their agreement with experimental data rely on the force field used to describe the system. In recent years, there have been several force fields proposed for sarin and its most common simulant dimethyl methylphosphonate (DMMP). However, other simulants frequently used in experiments received less attention from the molecular simulation perspective, for example, to date, there is no force field and no simulation data for diisopropyl methylphosphonate (DIMP). Here, we compare the literature force fields for sarin and DMMP, focusing specifically on the vapor-liquid equilibrium for the pure species. We carried out Monte Carlo and molecular dynamics simulations using the existing literature force fields from which we predicted the liquid densities and vapor pressures developing the entire binodal curves. We compared the predictions to the experimental data and showed that the TraPPE-UA force field outperformed the other force fields. Thus, we extended TraPPE-UA for DIMP, utilized this force field in molecular simulations, and predicted the liquid densities and vapor pressures for a range of temperatures (binodal curve), which agreed well with the published experimental data. From the binodal, we calculated the critical properties of DIMP and demonstrated that these parameters can be used in the Peng-Robinson equation of state for this compound.