Simultaneous photon absorption as a probe of molecular interaction and hydrogen-bond correlation in liquids

TL;DR

This study uses simultaneous photon absorption to probe molecular interactions and hydrogen-bond correlations in liquid methanol, revealing strong hydrogen-bond cooperativity and significant mode coupling.

Contribution

It introduces a quantitative method to measure hydrogen-bond correlation and mode coupling in liquids using simultaneous vibrational absorption spectroscopy.

Findings

RMS coupling strength of 46+/-1 cm-1 indicating strong interactions.

Hydrogen bonds in methanol are strongly correlated with a coefficient of 0.69+/-0.12.

Mode coupling exceeds what is expected from transition-dipole interactions.

Abstract

We have investigated the simultaneous absorption of near-infrared photons by pairs of neighboring molecules in liquid methanol. Simultaneous absorption by two OH-stretching modes is found to occur at an energy higher than the sum of the two absorbing modes. This frequency shift arises from interaction between the modes, and its value has been used to determine the average coupling between neighboring methanol molecules. We find a rms coupling strength of 46+/-1 cm-1, much larger than can be explained from transition-dipole coupling, suggesting that hydrogen-bond mediated interactions between neighboring molecules play an important role in liquid methanol. The most important aspect of simultaneous vibrational absorption is that it allows for a quantitative investigation of hydrogen-bond cooperativity. We derive the extent to which the hydrogen-bond strengths of neighboring molecules are correlated by comparing the line shape of the absorption band caused by simultaneous absorption with that of the fundamental transition. Surprisingly, neighboring hydrogen bonds in methanol are found to be strongly correlated, and from the data we obtain a hydrogen-bond correlation coefficient of 0.69+/-0.12.