Effects of hydrophobic solute on water normal modes

The vibrational modes of water get significantly modified by external solutes; this becomes particularly important when the solute is hydrophobic. In this work, we examine the effects of a tiny hydrophobe, methane, on the normal modes of water, using small cluster-based harmonic normal mode analysis of aqueous methane system. We estimate the vibrational density of states and also the infrared spectral density. We compare the methane-water data with the bulk water response. We decompose these modes based on different vibrational characters. The stretch-bend decomposition reflects a pronounced coupling between the methane asymmetric stretch mode and the water symmetric stretch mode. We examine the methane-water data in terms of the symmetry of the central water molecule's vibrations and find that asymmetric modes do not contribute. We also find that the vibrational modes having non-zero contributions from methane molecule are extremely localized in nature.