Changes in water structure induced by a hydrophobic solute probed by simulation of the water hydrogen bond angle and radial distribution functions.

In order to better characterize changes in water structure induced by a hydrophobic solute the oxygen-oxygen and hydrogen-hydrogen radial distribution functions (goo(r), ghh(r)) and the hydrogen bond angle distribution function p(theta) for water molecules in the first hydration shell of the tetramethyl ammonium (TMA) cation were computed using Monte Carlo simulations. goo(r) and ghh(r) were corrected for the effect of solute volume exclusion on the local solvent density so that intrinsic structural changes independent of local solvent density variations could be detected. Comparison of ghh(r) of TMA's first hydration shell water with ghh(r) for bulk water shows subtle but clear evidence of structure formation induced by the ion. These changes in ghh(r) are very similar to those seen experimentally for larger tetra-alkyl ammonium ions in previous neutron diffraction experiments. Larger changes in p(theta) in the first hydration shell of TMA were seen. Comparison of changes in p(theta) with changes in goo(r) and ghh(r) show that the angle distribution function provides the most sensitive way to analyze water structure changes associated with hydrophobic solvation.