(218c) Solvation Free Energies and 1-Octanol/Water Partition Coefficients of Nitroaromatic Compounds Using Molecular Simulation Techniques

Nitroaromatic compounds (NACs) have long been used as explosives, solvents, herbicides, insecticides, dyes and polyurethane foams in industry. These compounds are environmental contaminants and xenobiotics, and are largely distributed water mediated transportation. The extent of solvation of the NACs affects the level of contamination in soil, sediment, and ground water.  The properly model environmental impact of these nitroaromatic compounds, the solubility and most importantly the solvation free energies in water and in 1-octanol leading to the octanol-water partition coefficients are key components. We have calculated solvation free energies of a set of nitroaromatic compounds that includes multifunctional nitro groups using expanded ensemble molecular dynamics simulation algorithm. TIP3P and SPC-Fw water models have been used as the explicit solvent models in our simulations, and it has been found that the choice of the water model has a significant effect on the Gibbs solvation free energy simulations in water. GAFF, CGenFF, OPLS-AA and TraPPE-UA force fields parameters have been used to model the nitroaromatic compounds either with the pre-optimized partial charges for atom types, with Chelpg charges obtained from quantum calculations or with AM1-BCC charges estimated from a semi-empirical method. The GAFF and CGenFF force field parameters with Chelpg charges are found to reproduce experimental data relatively well with TIP3P water model. OPLS-AA force field performs equally well with the GAFF/CGenFF force fields, even with the pre-optimized partial charges assigned with atom types. The performance of different force fields were analyzed based on the number of nitro groups and it was found that most of the force fields studied here are somewhat limited in their ability to accurately predict the solvation behavior of  NACs with multiple nitro groups.