Hydrogen bonds in liquid water are broken only fleetingly

Abstract

Although it is widely accepted that the local structure of liquid water has tetrahedral arrangements of molecules ordered by hydrogen bonds, the mechanism by which water molecules switch hydrogen-bonded partners remains unclear. In this mechanism, the role of nonhydrogen-bonded configurations (NHBs) between adjacent molecules is of particular importance. A molecule may switch hydrogen-bonding partners either (i) through thermally activated breaking of a hydrogen bond that creates a dangling hydrogen bond before finding a new partner or (ii) by infrequent but rapid switching events in which the NHB is a transition state. Here, we report a combination of femtosecond 2D IR spectroscopy and molecular dynamics simulations to investigate the stability of NHB species in an isotopically dilute mixture of HOD in D 2O. Measured 2D IR spectra reveal that hydrogen-bonded configurations and NHBs undergo qualitatively different relaxation dynamics, with NHBs returning to hydrogen-bonded frequencies on the time scale of water's fastest intermolecular motions. Simulations of an atomistic model for the OH vibrational spectroscopy of water yield qualitatively similar 2D IR spectra to those measured experimentally. Analysis of NHBs in simulations by quenching demonstrates that the vast majority of NHBs are in fact part of a hydrogen-bonded well of attraction and that virtually all molecules return to a hydrogen-bonding partner within 200 fs. The results from experiment and simulation demonstrate that NHBs are intrinsically unstable and that dangling hydrogen bonds are an insignificant species in liquid water.femtosecond 2D IR spectroscopy ͉ molecular dynamics ͉ liquids O n average, molecules in liquid water are tetrahedrally coordinated but appear to engage in 10% fewer hydrogen bonds than in ice. Support for this estimate comes broadly, from latent heats of melting and vaporization, from x-ray and neutron scattering, and in very detailed form from molecular dynamics (MD) simulations (1-3). The role of nonhydrogen-bonded configurations (NHBs) in water's rapidly changing structure remains uncertain, lying at the heart of differences between mixture and continuum models of water (1, 3-8). Implicitly or explicitly, the interpretation of many experiments and MD simulations conceives of NHBs as broken or dangling hydrogen bonds, stable species that interconvert with a hydrogen-bonded configuration (HB) at a rate determined by the free energy barrier separating them. But it is also possible that NHBs are intrinsically unstable species that appear transiently during natural fluctuations about a hydrogen bond or when molecules trade hydrogen-bonding partners. These two scenarios not only provide qualitatively different interpretations of water's structure and how it evolves, but also imply different pictures for how water mediates chemical and biological processes. We have distinguished between these scenarios by using a combination of femtosecond 2D IR spectroscopy and MD simulations, finding that NHBs are inherently unstable, reforming ...

Two-Dimensional Infrared Spectroscopy of Isotope-Diluted Low Density Amorphous Ice

“…1), but the width of the distribution of hydrogen bond strengths as well as the amount of inhomogeneous broadening is comparable. The latter is deduced from the tilt of the 2D IR lineshape, which reflects the correlation of pump-and probe frequencies, and which has been observed also for liquid water at early enough population times [28][29][30][31][32]. However, water molecules are highly mobile in liquid water and exchange hydrogen bond partners on a 1 ps timescale.…”

“…Spectral diffusion is a main mechanism for such lineshape changes in liquid water [28][29][30][31][32]), but the glassy state assures that there is no rearrangement of hydrogen bonds on the timescale of the experiment.…”

“…Nonlinear vibrational spectroscopy, such as two dimensional infrared spectroscopy (2D IR) [26][27][28][29][30][31][32][33], is an excellent tool to monitor the local structure and dynamical properties of the hydrogen bonds in water. These experiments make use of the fact that the vibrational frequency of the OH stretch vibration is highly correlated with the OO distance of a pair of hydrogen bonded water molecules [34][35][36][37][38].…”

Microscopic properties of liquid water from combined ab initio molecular dynamics and energy decomposition studies

“…Although the thermal motion causes distortions from the perfectly tetrahedral configuration, each molecule in the liquid is bonded, on average, to the four nearest neighbours via two donor and two acceptor bonds 142 [145][146][147][148] , macroscopic thermodynamics data 142,143,149 as well as molecular dynamics simulations 105,108,143,144,150,151 .…”

Microscopic structure and dynamics of air/water interface by computer simulations—comparison with sum-frequency generation experiments

“…Although the present water model, TIP3P, may be satisfactory for predicting dangling surface OH bonds, there are indications from unpublished work that it predicts too many dangling OHs in bulk water, compared with experiments for non-bonded OH bonds. Measurements of non-bonded OHs in bulk water have been reported by Eaves et al 46 For studies of bulk reaction dynamics, an alternative to the TIP3P model should be explored. We are investigating how the SPC/E model, 47 which has been used for bulk water structure, will alter the calculated susceptibilities.…”