Effects of hydrophobic solute on water normal modes

TL;DR

This study investigates how a small hydrophobic molecule, methane, influences the vibrational normal modes of water, revealing mode coupling, localization, and spectral changes through harmonic analysis.

Contribution

It provides detailed insights into the vibrational mode modifications caused by methane in water, using cluster-based harmonic normal mode analysis, which is a novel approach for this system.

Findings

Pronounced coupling between methane asymmetric stretch and water symmetric stretch modes.

Vibrational modes involving methane are highly localized.

Asymmetric water modes do not significantly contribute to methane-water interactions.

Abstract

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.