Hydrogen bond relaxation dynamics and the associated vibronic and volumetric anomalies of H2O upon frozen

TL;DR

This study combines spectroscopy and simulations to clarify how hydrogen bond dynamics cause water's volume expansion and mechanical anomalies when frozen, revealing the roles of bond length changes and electron polarization.

Contribution

It provides a detailed molecular-level explanation of water's anomalous properties upon freezing, integrating the extended Ice Rule with experimental and computational data.

Findings

Intramolecular bonds contract upon cooling, following regular rules.

Intermolecular hydrogen bonds elongate and stiffen due to polarization effects.

Volume expansion is linked to nonbond elongation and electron polarization.

Abstract

A combination of the extended Ice Rule of Pauling, Raman spectroscopy, and molecular dynamics calculations has enabled us to clarify the bonding origin of the anomalous volume expansion, Raman phonon relaxation, and the stiffness and fragility of H2O upon frozen. We found that the initially shorter-and-stronger intramolecular "H+/p-O2-" bond follows the regular rule of cooling-contraction while the initially longer-and-weaker intermolecular "O2- : H+/p" nonbond turns to be even longer yet stiffer in the "O2- : H+/p-O2-" hydrogen-bond of H2O upon frozen, as a consequence of the polarization and Coulomb repulsion between the unevenly-bounded bonding and nonbonding electron pairs. The elongation of the nonbond and the polarization of the nonbonding lone pair are responsible, respectively, for the volume expansion and the stiffness and the fragility of ice. Findings should form important impact to the understanding of the physical anomalies of H2O under other stimuli such as pressure and confinement.