Universal Hydrogen Bond Symmetrisation Dynamics Under Extreme Conditions

TL;DR

This study uses high-resolution extit{in-situ} $^1$H-NMR to investigate hydrogen bond symmetrisation under extreme pressures up to 90 GPa, revealing universal dynamics and critical distances associated with hydrogen mobility and localisation.

Contribution

It provides the first direct NMR evidence of hydrogen bond symmetrisation dynamics across various systems under high pressure, identifying a universal critical O--O distance.

Findings

Minima in NMR line-widths indicate maximum hydrogen mobility before localisation.

Critical O--O distance for symmetrisation is approximately 2.443 Å.

Hydrogen mobility peaks at specific pressures regardless of chemical environment.

Abstract

The experimental study of hydrogen bonds and their symmetrisation under extreme conditions is predominantly driven by diffraction methods, despite challenges of localising or probing the hydrogen subsystems directly. Until recently, H-bond symmetrisation has been addressed in terms of either nuclear quantum effects, spin crossovers or direct structural transitions; often leading to contradictory interpretations when combined. Here, we present high-resolution \textit{in-situ} $^1$H-NMR experiments in diamond anvil cells investigating a wide range of hydrogen bonded systems at pressure ranges of up to 90 GPa covering their respective H-bond symmetrisation. We found pronounced minima in the pressure dependence of the NMR resonance line-widths associated with a maximum in hydrogen mobility, precursor to a localisation of hydrogen atoms. These minima, independent of of the chemical environment of the linear O -- H-O unit, can be found in a narrow range of oxygen-oxygen distances between 2.44 and 2.45 \AA, leading to an average critical oxygen-oxygen distance of $\bar{r}_{\rm OO}^{crit}=2.443(1)$ \AA.