Anisotropic structure and dynamics of the solvation shell of a benzene solute in liquid water from ab initio molecular dynamics simulations

The anisotropic structure and dynamics of the hydration shell of a benzene solute in liquid water have been investigated by means of ab initio molecular dynamics simulations using the BLYP (Becke-Lee-Yang-Parr) and dispersion corrected BLYP-D functionals. The main focus has been to look at the influence of pi-hydrogen-bonding and hydrophobic interactions on the distance and angle resolved various structural and dynamic properties of solvation shell. The structure of hydration shell water molecules around benzene is found to be highly anisotropic as revealed by the radial distribution functions of different conical regions and joint radial/angular distribution functions. The benzene water dimer potential energy curves are calculated for a variety of orientations of water along the axial and equatorial directions for both BLYP and BLYP-D functionals. The simulation results of the hydration shell structure of benzene, particularly the axial and equatorial benzene water RDFs are discussed based on the differences in the benzene water potential energies for different orientations and functionals. The inter-particle distance/angle correlations show an enhanced water structure in the solvation shell of benzene compared to that between the solvation shell and bulk and also between the bulk molecules. On average, a single pi H-bond is found to be formed between water and benzene in the 45 axial conical region of the solvation shell. The pi H-bonded water molecules are found to have faster translational dynamics and also found to follow a fast jump mechanism of reorientation to change their hydrogen bonded partners. The presence of pi-hydrogen-bonded water makes the overall dynamics of the axial region faster than that of the equatorial region where the water molecules are hydrophobically solvated and hydrogen bonded to other water molecules.