Theoretical Spectroscopic Investigation of Hydrogen Bonding and Hydrophobicity

TL;DR

This study uses theoretical spectroscopy to analyze how methane affects water's hydrogen bonding network, revealing microscopic details of solvation structures and their temperature dependence through vibrational spectral maps.

Contribution

It introduces a mixed quantum-classical simulation approach to map vibrational spectra and connect them to molecular solvation structures around methane.

Findings

Presence of gas-like water molecules in methane hydration shell.

Significant increase in ordering of first solvation water, except facing molecules.

Temperature influences water molecules' behavior near methane.

Abstract

Hydrophobic solutes significantly alters water hydrogen bond network. The local alteration of solvation struc-tures get reflected in the vibrational spectroscopic signal. Although it is possible to detect this microscopicfeatures by modern infrared spectroscopy, however, bulk phase spectra often comes with formidable challengeof establishing the connection among the experimental spectra to molecular structures. Theoretical spec-troscopy can serve as more powerful tool even where spectroscopic data cannot provide microscopic picture.In the present work, we build a theoretical spectroscopic map based on mixed quantum-classical molecularsimulation approach using methane in water system. The single oscillator level O-H stretch frequency is wellcorrelated with a collective variable solvation energy. We construct the spectroscopic maps for fundamentaltransition frequencies and also the transition dipoles. A bimodal frequency distribution with a blue shiftedpopulation of transition frequency illustrates presence of gas like water molecules in the hydration shell ofmethane. This observation is further complemented by a shell-wise decomposition of the O-H stretch fre-quencies. We observe a significant increase in ordering of the first solvation water except the water molecules,which are directly facing the methane molecule. This is manifested in redshift of the observed transition fre-quencies. Temperature dependent simulations depict that the water molecules facing to the methane moleculebehave similar to the high temperature water and the rest of the first shell water molecules behave more likecold water.