There has been considerable interest in the development of new Insensitive Munitions (IM) compounds due to their thermal stability and low shock sensitivity over traditionally used explosive compounds. Six such compounds are 2,4-dinitroanisole (DNAN), N-methyl-p-nitroaniline (MNA), Dinitropyrazole (DNP), Nitrotriazolone (NTO), 1-methyl-2,4,5-trinitroimidazole (MTNI) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Due to the increased environmental and safety concerns, it is necessary to determine how these compounds behave in the environment in terms of bioaccumulation potential and aqueous solubility. A pre-biological screen [1] based on the knowledge of octanol-water partition coefficients (log K_ow) and Henry's law constants (log H) is useful in this aspect. Atomistic simulation is attractive as a predictive tool for assessing the environmental impact of these compounds due to their hazardous nature. An accurate atomic force field with an appropriate methodology is the key to predicting accurate thermophysical properties.

In this work, we develop molecular models, or “force fields” for the explosives to predict their octanol-water partition coefficients, Henry's law constants and also vapor-liquid equilibria, vapor pressure and critical points. For the non-bonded interactions, Lennard-Jones parameters for each interaction site are obtained from the TraPPE-UA [2,3,4] and TraPPE-EH force fields [5] from similar functional groups. The partial charges are fitted to reproduce partition coefficients and Henry's law constants from the experiment. NPT molecular dynamics simulations coupled with the free energy perturbation technique is used to compute the physicochemical properties and Gibbs-Duhem integration is used to determine the vapor-liquid equilibria, vapor pressure and critical parameters. The log K_ow values predicted for DNAN and MNA are 1.648 and 1.934, respectively and are within 3 % of experiment.

[1] J.W.Gillett, Environmental Toxicology and Chemistry. 2, 463 (1983).

[2] C.D.Wick, J.M.Stubbs, N.Rai, J.I.Siepmann, J.Phys.Chem.B, 109 (18974).

[3] M.G.Martin, J.I.Siepmann, J.Phys.Chem.B, 104 (8008).

[4] J.M.Stubbs, J.J.Potoff, J.I.Siepmann, J.Phys.Chem.B, 108 (17596).

[5] N.Rai, J.I.Siepmann, J.Phys.Chem.B, 111 (2007).