Nuclear quantum effects and hydrogen bond fluctuations in water

Significance There is no doubt about the importance of liquid water for climate and life on Earth. Correctly modeling the properties of this substance is still a formidable challenge, however. Here, we show, using state-of-the-art techniques that allow for quantum mechanical effects in the motion of the electrons and nuclei, that room-temperature water is not simply a molecular liquid; its protons experience wild excursions along the hydrogen bond (HB) network driven by quantum fluctuations, which result in an unexpectedly large probability of transient autoionization events. Moreover, these events are strongly correlated across neighboring bonds so that perturbations disrupting the HB network (pressure, confinement, solvated ions, and interfaces) could enhance in a concerted way their impact on water’s behavior. The hydrogen bond (HB) is central to our understanding of the properties of water. However, despite intense theoretical and experimental study, it continues to hold some surprises. Here, we show from an analysis of ab initio simulations that take proper account of nuclear quantum effects that the hydrogen-bonded protons in liquid water experience significant excursions in the direction of the acceptor oxygen atoms. This generates a small but nonnegligible fraction of transient autoprotolysis events that are not seen in simulations with classical nuclei. These events are associated with major rearrangements of the electronic density, as revealed by an analysis of the computed Wannier centers and 1H chemical shifts. We also show that the quantum fluctuations exhibit significant correlations across neighboring HBs, consistent with an ephemeral shuttling of protons along water wires. We end by suggesting possible implications for our understanding of how perturbations (solvated ions, interfaces, and confinement) might affect the HB network in water.