High-temperature pyrolysis experiments and chemical kinetics of diisopropyl methylphosphonate (DIMP), a simulant for Sarin

Abstract

Due to the high toxicity of chemical warfare (CW) agents, laboratory experiments are carried out using CW surrogates. Diisopropyl Methylphosphonate (DIMP), an organophosphate compound (OPC), is a crucial CW surrogate that has a chemical structure similar to the deadly nerve agent Sarin (GB). In this work, high-temperature pyrolysis of DIMP is conducted in a shock tube within a temperature range of 1400 - 1800 K at near 1 atm pressure. Laser absorption spectroscopy was used to obtain carbon monoxide mole fraction time-histories during high-temperature pyrolysis for DIMP. In order to predict the CO mole fraction time-histories during DIMP pyrolysis, a reaction mechanism was developed using the Lawrence Livermore National Lab (LLNL)’s OPC mechanism. Several important reactions of DIMP and its important intermediates during pyrolysis were studied at the CBS-QB3 level of theory, and corresponding reaction rates were determined using transition state theory. The reactions related to methyl phosphonic acid (MPA) and methyl(oxo)phosphoniumolate (MOPO) were added from literature and by analogies with similar compounds. Thermochemical properties of crucial species involved in DIMP pyrolysis were calculated using the CBS-QB3 composite method and were updated in the model. A reaction mechanism was compiled for DIMP pyrolysis using these rates. Two versions of this mechanism (‘Model A’ and ‘Model B’) were generated to understand the significance of new reactions in predicting CO mole fraction time histories. Significant improvement in the prediction of CO was observed compared to the LLNL model with either of these mechanisms. A reaction path analysis was conducted to understand the formation pathways of CO during DIMP pyrolysis. Finally, sensitivity analysis was performed to get insights into important reactions leading to CO formation. It was found that the H-abstraction from MOPO by methyl radical plays an important role in CO formation.