Nuclear quantum effects and hydrogen bond fluctuations in water

Abstract

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 1 H 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.path integral molecular dynamics | generalized Langevin equation thermostat | ab initio liquid water D espite its apparent simplicity, liquid water exhibits a number of anomalous properties, such as a decrease in density on freezing, an isobaric density maximum, and its unusually high dielectric constant and heat capacity (1). These, together with its unquestionable importance for climate and life on Earth, have made this substance a subject of intense research by both experiments and simulations.The central concept that has been used to rationalize the peculiar behavior of water is that of the hydrogen bond (HB) (2). The nature of this bond in water has been studied in depth by atomistic computer simulations, which have investigated how it is affected by ionic and electronic polarizability (3, 4), pressure and temperature (5, 6), and nuclear quantum effects (NQEs) (7-9). Furthermore, ab initio molecular dynamics (MD) simulations have been used to shed light on autoionization (10, 11), a process with profound implications for the chemistry of aqueous solutions.In this paper, we investigate the impact of NQEs on the HB in pure water, finding a qualitative increase in fluctuations that leads to a partial dissociation of the covalent O-H bond. The weakening of this covalent bond in the presence of hydrogen bonding is consistent with the red shift of the stretching mode of water upon condensation, as well as with recent experiments demonstrating that selectively exciting the O-H stretch in water leads to a pronounced delocalization of the proton toward the acceptor oxygen atom (12). However, the role of NQEs in governing the extent of this delocalization has not been investigated before now.Our analysis is based on MD simulations of water at different thermodynamic state points, with an ab initio description of the interactions among the nuclei. We also account fully for the quantum nature of the nuclear motion, using a recently developed combination of imaginary time path i...

Ab initio Electronic Structure of Liquid Water

“…Thus, thermal fluctuations alone do not suffice to recover the band-gap renormalization found with the quantum nuclei. Indeed, we find that the proton-transfer coordinate distribution [28] at 400 K is only marginally broadened compared to what has been observed at 300 K (see the Supplemental Material [49]), and there is no sign of a delocalized hydrogen-bond distribution around δ OH ¼ 0. Figure 2 (inset) further reveals that the temperature dependence is considerably suppressed in the presence of quantum fluctuations.…”

“…The inclusion of nuclear quantum effects (NQEs) noticeably modifies the structural and dynamical properties of liquid water [23][24][25][26][27][28][29]. In the context of the electronic structure, the quantum zero-point motion of nuclei renormalizes the electronic band gap [30][31][32][33][34][35][36][37][38][39].…”

High order path integrals made easy

“…In order to probe the behavior of our approach in a lower-temperature regime, we also performed simulations of a 96-molecules box of hexagonal ice at T = 100 K. For colored-noise simulations we used the PIGLET thermostat 20 for Trotter PI, and the SC+GLE strategy discussed above for SC PIMD. These calculations will be a challenging test case for our techniques, because the reactive nature of the NN potential allows for quantum fluctuations of the hydrogen bond probing the strongly anharmonic regions in the potential energy surface of water 49 .…”

“…One of the most remarkable effects of quantum fluctuations in room-temperature water is the occurrence of transient self-dissociation events, in which a quantum fluctuation momentarily brings a proton closer to the acceptor oxygen atom than to the oxygen it is covalently bound to 49 . The extent of these fluctuations is a particularly challenging quantity to compute, because of the small fraction of particles that undergo such broad excursions at any given time, the strong anharmonicity of the potential in this region, and the dependence on the level of electronic structure theory 52 .…”

Effect of quantum nuclear motion on hydrogen bonding

“…Most of chemistry can be understood in terms of semi-classical motion of nuclei on potential energy surfaces. In contrast, the quantum dynamics of protons involved in hydrogen bonds plays an important role in liquid water [1][2][3], ice [4,5], transport of protons and hydroxide ions in water [6], surface melting of ice [7], the bond orientation of water and isotopic fractionation at the liquid-vapour interface [8], isotopic fractionation in water condensation [9], proton transport in water-filled carbon nanotubes [10], hydrogen chloride hydrates [11], proton sponges [12,13], water-hydroxyl overlayers on metal surfaces [14], and in some proton transfer reactions in enzymes [15]. Experimentally, the magnitude of these nuclear quantum effects are reflected in isotope effects, where hydrogen is replaced with deuterium.…”