Electronic signature of the instantaneous asymmetry in the first coordination shell of liquid water

TL;DR

This study reveals that liquid water exhibits significant instantaneous asymmetry in hydrogen bonding, which explains X-ray spectral features and reconciles symmetric and asymmetric structural models.

Contribution

The paper introduces a novel energy decomposition method to analyze local interactions in water, highlighting the role of instantaneous asymmetry in hydrogen bonds.

Findings

Water molecules have fluctuating asymmetric hydrogen bonds.

X-ray spectra are influenced by molecules with high instantaneous asymmetry.

Instantaneous distortions occur on a timescale of hundreds of femtoseconds.

Abstract

Interpretation of the X-ray spectra of water as evidence for its asymmetric structure has challenged the conventional symmetric nearly-tetrahedral model and initiated an intense debate about the order and symmetry of the hydrogen bond network in water. Here, we present new insights into the nature of local interactions in water obtained using a novel energy decomposition method. Our simulations reveal that while a water molecule forms, on average, two strong donor and two strong acceptor bonds, there is a significant asymmetry in the energy of these contacts. We demonstrate that this asymmetry is a result of small instantaneous distortions of hydrogen bonds, which appear as fluctuations on a timescale of hundreds of femtoseconds around the average symmetric structure. Furthermore, we show that the distinct features of the X-ray absorption spectra originate from molecules with high instantaneous asymmetry. Our findings have important implications as they help reconcile the symmetric and asymmetric views on the structure of water.