Hiding in the crowd: Spectral signatures of overcoordinated hydrogen bond environments

TL;DR

This study combines experimental Raman spectroscopy and machine learning to identify overcoordinated hydrogen bond environments in water, revealing their spectral signatures and temperature-dependent behavior, advancing understanding of complex hydrogen bond networks.

Contribution

It introduces a novel approach integrating Raman measurements and machine learning to detect overcoordinated hydrogen bonds in water, enhancing spectral interpretation and understanding of hydrogen bond networks.

Findings

Overcoordinated hydrogen bonds have distinct spectral signatures.

Spectral signatures of overcoordinated bonds persist across temperature regimes.

Population of these bonds shows a temperature-dependent turnover.

Abstract

Molecules with an excess number of hydrogen-bonding partners play a crucial role in fundamental chemical processes, ranging from the anomalous diffusion in supercooled water to the transport of aqueous proton defects and the ordering of water around hydrophobic solutes. Here we show that overcoordinated hydrogen bond environments can be identified in both the ambient and supercooled regimes of liquid water by combining experimental Raman multivariate curve resolution measurements and machine learning accelerated quantum simulations. In particular, we find that OH groups appearing in spectral regions usually associated with non-hydrogen-bonded species actually correspond to hydrogen bonds formed in overcoordinated environments. We further show that only these species exhibit a turnover in population as a function of temperature, which is robust and persists under both constant pressure and density conditions. This work thus provides a new tool to identify, interpret, and elucidate the spectral signatures of crowded hydrogen bond networks.