CHARMM Force Field Parameters for Nitroalkanes and Nitroarenes.

New CHARMM force field (FF) parameters are developed for nitro compounds, referred to here as C27rn, for subsequent use in molecular dynamics (MD) simulations. The nonbonded terms are adjusted to best fit densities and hydration energies of nitropropane and nitrobenzene. High-level quantum mechanical calculations are used to obtain accurate conformational energies of nitroalkanes and nitrobenzene and to adjust the torsional potential of the CHARMM FF. For nitroalkanes, the calculated gauche (g) conformer of the C-C-C-N torsion is more stable than trans (t). Consequently, nitropropane MD simulations with C27rn result in 74% population of this g conformer. The C27rn FF is in excellent agreement with experiment for various bulk (density, isothermal compressibility, and heat of vaporization) and interfacial (surface tension) properties of nitropropane, nitrobutane, and nitrobenzene. MD simulations with the OPLS-AA FF for nitropropane and nitrobenzene result in similar property predictions as C27rn, except a reduced stability of the C-C-C-N g conformer.