Direct observation of ultrafast hydrogen bond strengthening in liquid water

Abstract

Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network 1 , including unusually efficient vibrational energy redistribution and relaxation 2 . An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments 2-7 . However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here we measured the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04 Å on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of ~1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on the femtosecond timescales. Our experiment and simulations unveiled the intermolecular character of the water vibration preceding the relaxation of the OH stretch.The vibrational spectroscopy of water has critically contributed to our understanding of the dynamics of hydrogen bond (HB) reorganization and energy redistribution in liquid water. It is

Using Car-Parrinello simulations and microscopic order descriptors to reveal two locally favored structures with distinct molecular dipole moments and dynamics in ambient liquid water

“…The observation of structural fluctuations in ambient liquid water via DFT-based MD simulations is consistent with recent experiments 45 on water ultrafast vibrational motion, pointing out the importance of the quantum mechanical nature of water hydrogen bonds at ambient conditions. Here, our main aim is to study the dynamics of the water molecules in the liquid state, calculating the local structural descriptor proposed by Russo and Tanaka with quantum mechanical methodologies to bring more profound insight into water's peculiar properties.…”

A Benchmark Protocol for DFT Approaches and Data-Driven Models for Halide-Water Clusters

“…The study of water’s properties is one of the most intense research topics [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. In practice, you can rarely find water in its pure state, and the presence of other molecules is common, which can produce changes in the observed behavior of the water network.…”

Path Integral Molecular Dynamics of Liquid Water in a Mean‐Field Particle Reservoir

“…Classical molecular dynamics over the last decades has proven to be a powerful tool of investigation for solvation processes on a large scale [1] while ab initio molecular dynamics has provided a deep understanding of the microscopic dynamics of hydrogen bonds of water at quantum mechanical/electronic level. [2] However, while classical models have the inherent limit of not taking explicitly into account the quantum nature of the hydrogen bonding, ab initio molecular dynamics is often computationally too demanding for the scale of analysis required by many systems of interest. Path integral molecular dynamics (PIMD),[ 3 , 4 ] using classical‐like potentials, lies in between, in terms of both: computational costs and increase of physical accuracy.…”