Solvation free energies and hydration structure of N-methyl-p-nitroaniline

Solvation Gibbs energies of N-methyl-p-nitroaniline (MNA) in water and 1-octanol are calculated using the expanded ensemble molecular dynamics method with a force field taken from the literature. The accuracy of the free energy calculations is verified with the experimental Gibbs free energy data and found to reproduce the experimental 1-octanol∕water partition coefficient to within ±0.1 in log unit. To investigate the hydration structure around N-methyl-p-nitroaniline, an independent NVT molecular dynamics simulation was performed at ambient conditions. The local organization of water molecules around the solute MNA molecule was investigated using the radial distribution function (RDF), the coordination number, and the extent of hydrogen bonding. The spatial distribution functions (SDFs) show that the water molecules are distributed above and below the nitrogen atoms parallel to the plane of aromatic ring for both the methylamino and nitro functional groups. It is found that these groups have a significant effect on the hydration of MNA with water molecules forming two weak hydrogen bonds with both the methylamino and nitro groups. The hydration structures around the functional groups in MNA in water are different from those that have been found for methylamine, nitrobenzene, and benzene in aqueous solutions, and these differences together with weak hydrogen bonds explain the lower solubility of MNA in water. The RDFs together with SDFs provide a tool for the understanding the hydration of MNA (and other molecules) and therefore their solubility.