HYDROGEN bonds play a crucial role in the behaviour of water1–4; their spatial patterns and fluctuations characterize the structure and dynamics of the liquid5–7. The processes of breaking and making hydrogen bonds in the condensed phase can be probed indirectly by a variety of experimental techniques8, and more quantitative information can be obtained from computer simulations9. In particular, simulations have revealed that on long timescales the relaxation behaviour of hydrogen bonds in liquid water exhibit non-exponential kinetics7,10–13, suggesting that bond making and breaking are not simple processes characterized by well defined rate constants. Here we show that these kinetics can be understood in terms of an interplay between diffusion and hydrogen-bond dynamics. In our model, which can be extended to other hydrogen-bonded liquids, diffusion governs whether a specific pair of water molecules are near neighbours, and hydrogen bonds between such pairs form and persist at random with average lifetimes determined by rate constants for bond making and breaking.

Hydrogen-bond kinetics in liquid water

Despite long study, a molecular picture of the mechanism of water reorientation is still lacking Using numerical simulations, we find support for a pathway in which the rotating water molecule breaks a hydrogen bond (H-bond) with an overcoordinated first-shell neighbor to form an H-bond with an undercoordinated second-shell neighbor The H-bond cleavage and the molecular reorientation occur concertedly and not successively as usually considered This water reorientation mechanism involves large-amplitude angular jumps, rather than the commonly accepted sequence of small diffusive steps, and therefore calls for reinterpretation of many experimental data wherein water rotational relaxation is assumed to be diffusive

https://science.sciencemag.org/content/sci/311/5762/832.full.pdf

A molecular jump mechanism of water reorientation.

We investigated rearrangements of the hydrogen-bond network in water by measuring fluctuations in the OH-stretching frequency of HOD in liquid D2O with femtosecond infrared spectroscopy. Using simulations of an atomistic model of water, we relate these frequency fluctuations to intermolecular dynamics. The model reveals that OH frequency shifts arise from changes in the molecular electric field that acts on the proton. At short times, vibrational dephasing reflects an underdamped oscillation of the hydrogen bond with a period of 170 femtoseconds. At longer times, vibrational correlations decay on a 1.2-picosecond time scale because of collective structural reorganizations.

https://science.sciencemag.org/content/sci/301/5640/1698.full.pdf

Ultrafast hydrogen-bond dynamics in the infrared spectroscopy of water.

Femtosecond visible and infrared analogues of multiple-pulse nuclear magnetic resonance techniques provide novel snapshot probes into the structure and electronic and vibrational dynamics of complex molecular assemblies such as photosynthetic antennae, proteins, and hydrogen-bonded liquids. A classical-oscillator description of these spectroscopies in terms of interacting quasiparticles (rather than transitions among global eigenstates) is developed and sets the stage for designing new pulse sequences and inverting the multidimensional signals to yield molecular structures. Considerable computational advantages and a clear physical insight into the origin of the response and the relevant coherence sizes are provided by a real-space analysis of the underlying coherence-transfer pathways in Liouville space.

Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations.

Theoretical descriptions of two-dimensional (2D) vibrational and electronic spectroscopy are presented By using a coupled multi-chromophore model, some examples of 2D spectroscopic studies of peptide solution structure determination and excitation transfer process in electronically coupled multi-chromophore system are discussed A few remarks on perspectives of this research area are given

Coherent Two-Dimensional Optical Spectroscopy

A pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5– 4.4 mm spectral region, at a 10 mJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the OH · · · O motions are “photographed” in real time. [S0031-9007(98)08298-2]

Femtosecond dynamics of hydrogen bonds in liquid water : a real time study

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH⋯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump–probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH⋯O bonds in water is rotation–limited.

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Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment

A theory is developed to describe a recent infrared pump–probe experiment in water. This theory is a statistical theory, similar to those elaborated earlier to interpret ultraviolet and visible spectra. Nonlinear optical processes involved are analyzed in terms of four‐time correlation functions of the electric dipole moment of the system and of the incident electric fields, respectively. An analytical expression for the transient infrared signal is presented. The observed spectral characteristics are attributed to a gradual thermalization of the initial, pump‐prepared state. The substructure of the observed bands is interpreted. This experiment probes the kinetics of conversion of H‐bonds of different lengths into each other.

Ultrafast infrared pump–probe spectroscopy of water: A theoretical description

The pump–probe response of water in infrared is examined theoretically at time scales of the order of 100 fs; these times scales are characteristic of spectral diffusion. The theory is a statistical theory using correlation function description of the nonlinear optical processes involved. An analytical expression for the transient infrared signal is presented, and the corresponding time‐ and frequency‐resolved spectra are discussed. Real time shifts of spectral bands, preceded by a redistribution of their intensities, are predicted. Physical interpretation of these findings is proposed, corroborated by the help of simple models. The intrinsic interest of this time domain is emphasized.

J-Cl. Leicknam

Papers

Femtosecond dynamics of hydrogen bonds in liquid water : a real time study

Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment

Ultrafast infrared pump–probe spectroscopy of water: A theoretical description

Subpicosecond transient infrared spectroscopy of water : a theoretical description

Non-monotonic decay of transient infrared absorption in dilute HDO/D2O solutions