Hydrogen Bond Network In Liquid Water Research Articles

Enhancement of the local asymmetry in the hydrogen bond network of liquid water by an ultrafast electric field pulse

Condensed phase electron decomposition analysis based on density functional theory has recently revealed an asymmetry in the hydrogen-bond network in liquid water, in the sense that a significant population of water molecules are simultaneously donating and accepting one strong hydrogen-bond and another substantially weaker one. Here we investigate this asymmetry, as well as broader structural and energetic features of water’s hydrogen-bond network, following the application of an intense electric field square pulse that invokes the ultrafast reorientation of water molecules. We find that the necessary field-strength required to invoke an ultrafast alignment in a picosecond time window is on the order of 108 Vm−1. The resulting orientational anisotropy imposes an experimentally measurable signature on the structure and dynamics of the hydrogen-bond network, including its asymmetry, which is strongly enhanced. The dependence of the molecular reorientation dynamics on the field-strength can be understood by relating the magnitude of the water dipole–field interaction to the rotational kinetic energy, as well as the hydrogen-bond energy.

Shrinking of Rapidly Evaporating Water Microdroplets Reveals their Extreme Supercooling.

The fast evaporative cooling of micrometer-sized water droplets in a vacuum offers the appealing possibility to investigate supercooled water-below the melting point but still a liquid-at temperatures far beyond the state of the art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a train of perfectly uniform water droplets allows us to measure the variation in droplet size resulting from evaporative mass losses with an absolute precision of better than 0.2%. This finding proves crucial to an unambiguous determination of the droplet temperature. In particular, we find that a fraction of water droplets with an initial diameter of 6379±12 nm remain liquid down to 230.6±0.6 K. Our results question temperature estimates reported recently for larger supercooled water droplets and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.

Structure of liquid water - a dynamical mixture of tetrahedral and 'ring-and-chain' like structures.

The nature of the dynamical hydrogen-bond network of liquid water under ambient conditions has challenged both experimental and theoretical researchers for decades and remains a topic of intense debate. In this work, we addressed the structural issue of the hydrogen-bond network of liquid water based on an accurate ab initio molecular dynamics simulation. The present work showed clearly that liquid water is neither accurately described by a static picture of mostly tetrahedral water molecules nor dominated by "ring-and-chain" like structures. Instead, the structure of water is a dynamical mixture of tetrahedral and 'ring-and-chain' like structures with a slight bias toward the former. On average, each water molecule forms about three hydrogen bonds with the surrounding water molecules. The present accurate ab initio molecular dynamics simulation of liquid water was made possible by using a fragment-based second-order Møller-Plesset perturbation theory (MP2) with a large basis set to treat a large body of water molecules. This level of ab initio theory is sufficiently accurate for describing water interactions, and the simulated structural and dynamical properties of liquid water, including radial distribution functions, diffusion coefficient, dipole moment, etc., are uniformly in excellent agreement with experimental observations.

Structural, Dynamical, and Electronic Properties of Liquid Water: A Hybrid Functional Study.

We study structural, dynamical, and electronic properties of liquid water through ab initio molecular dynamics (MD) simulations based on a hybrid functional which includes nonlocal van der Waals (vdW) interactions. The water dimer, the water hexamer, and two phases of ice are studied as benchmark cases. The hydrogen-bond energy depends on the balance between Fock exchange and vdW interactions. Moreover, the energetic competition between extended and compact structural motifs is found to be well described by theory provided vdW interactions are accounted for. Applied to the hydrogen-bond network of liquid water, the dispersion interactions favor more compact structural motifs, bring the density closer to the experimental value, and improve the agreement with experimental observables such as radial distribution functions. The description of the self-diffusion coefficient is also found to improve upon the combined consideration of Fock exchange and vdW interactions. The band gap and the band edges are found to agree with experiment within 0.1 eV.

Femtosecond mid-infrared study of the reorientation of weakly hydrogen-bonded water molecules.

We study the hydrogen-bond and reorientation dynamics of weakly hydrogen-bonded water molecules by studying their spectral diffusion and anisotropy dynamics with polarization-resolved two-color femtosecond mid-infrared spectroscopy. We selectively excite weakly hydrogen-bonded water molecules by tuning a relatively narrow band excitation pulse far into the high-frequency wing of the O-D stretch vibration of HDO molecules in H2O water. We observe that the spectral diffusion and the anisotropy both show pronounced biexponential dynamics. On the basis of previous work, the fast component of the spectral dynamics with a time constant of ∼100 fs is assigned to rapid hydrogen-bond switching events. We observed that these switching events lead to a pronounced effect on the anisotropy of the excited O-D groups, which shows that the spectral relaxation is accompanied by a large change of the orientation of the O-D groups. The slow component of the spectral relaxation can be assigned to the collective structural reorganization of the hydrogen-bond network of liquid water. With increasing temperature, the spectral relaxation shows a similar acceleration as the average molecular reorientation, showing that these processes are intimately connected.

The Hydrogen Bond of Water from the Perspective of Soft X-Ray Spectroscopy

Its importance for life and its unusual properties keep water within the focus of ongoing research; this focus especially applies to water in the liquid phase. Scientists agree that the hydrogen-bond network, which is formed by interactions between the water molecules, is key for understanding the anomalies of water. However, a better understanding of the structure of this network, as well as its dynamics, must yet be established. Soft X-ray spectroscopy allows the investigation of the local electronic structure of water by probing the occupied and unoccupied valence molecular orbitals. In this Focus Review, we present soft-X-ray-based techniques, their development in terms of liquid spectroscopy, and recent studies on the hydrogen-bond network of liquid water.

On the Origin of the Hydrogen‐Bond‐Network Nature of Water: X‐Ray Absorption and Emission Spectra of Water–Acetonitrile Mixtures

A liquid microjet was used to obtain oxygen K-edge X-ray absorption and emission spectra of water–acetonitrile mixtures of various compositions. The observed spectral changes are unambiguously related to the increasing number of broken hydrogen bonds with decreasing water concentration, and the hydrogen-bond network of liquid water can thus be addressed on purely experimental grounds without the need for theoretical modeling.

Nuclear Quantum Effects in the Reorientation of Water

The molecular reorientation associated with the dynamics of the hydrogen-bond network in liquid water is investigated using quantum molecular dynamics simulations performed with the ab-initio-based TTM3-F interaction potential. The reorientation dynamics calculated at different temperatures are found to be in excellent agreement with the corresponding experimental results obtained from polarization-resolved, femtosecond mid-infrared, pump−probe spectroscopic measurements. A comparison with analogous results obtained from classical molecular dynamics simulations with the same interaction potential clearly indicates that the explicit inclusion of nuclear quantum effects is critical for reproducing the measured time dependence of the anisotropic signal.

The OH stretch vibration of liquid water reveals hydrogen-bond clusters

Using molecular dynamics simulations, we investigate the fluctuations of the hydrogen-bond network in liquid water and its relation to vibrational spectroscopy. We show that the high-frequency shoulder, which is most evidently found in Raman spectra of the OH stretch vibration of isotope diluted water, is (at least to a certain extent) related to a three-fold hydrogen-bonded ring. This suggests that it is not always sufficient to classify individual water molecules when studying some aspects of hydrogen-bond dynamics, rather, one should consider the topology of the local structure around a given water molecule.

Two-Dimensional Infrared Spectroscopy of Intermolecular Hydrogen Bonds in the Condensed Phase

Hydrogen bonding plays a key role in the structural, physical, and chemical properties of liquids such as water and in macromolecular structures such as proteins. Vibrational spectroscopy is an important tool for understanding hydrogen bonding because it provides a way to observe local molecular geometries and their interaction with the environment. Linear vibrational spectroscopy has mapped characteristic changes of vibrational spectra and the occurrence of new bands that form upon hydrogen bonding. However, linear vibrational spectroscopy gives very limited insight into ultrafast dynamics of the underlying molecular interactions, such as the motions of hydrogen-bonded groups, energy dissipation and delocalization, and the fluctuations within hydrogen-bonded structures that occur in the ultrafast time domain. Nonlinear vibrational spectroscopy with its femtosecond time resolution can discern these dynamic processes in real time and has emerged as an important tool for unraveling molecular dynamics and for quantifying interactions that govern the vibrational and structural dynamics of hydrogen bonds. This Account reviews recent progress originating from third-order nonlinear methods of coherent multidimensional vibrational spectroscopy. Ultrafast dynamics of intermolecular hydrogen bonds are addressed for a number of prototype systems: hydrogen-bonded carboxylic acid dimers in an aprotic liquid environment, the disordered fluctuating hydrogen-bond network of liquid water, and DNA oligomers interacting with water. Cyclic carboxylic acid dimers display a rich scheme of vibrational couplings, resulting in OH stretching absorption bands with highly complex spectral envelopes. Two-dimensional spectroscopy of acetic acid dimers in a nonpolar liquid environment demonstrates that multiple Fermi resonances of the OH stretching mode with overtones and combination tones of fingerprint vibrations dominate both the 2D and linear absorption spectra. The coupling of the OH stretching mode with low-frequency hydrogen-bonding modes leads to additional progressions and coherent low-frequency hydrogen-bond motions in the subpicosecond time domain. In water, the 2D spectra reveal ultrafast spectral diffusion on a sub-100 fs time scale caused by the ultrafast structural fluctuations of the strongly coupled hydrogen-bond network. Librational motions play a key role for the ultrafast loss of structural memory. Spectral diffusion rates are enhanced by resonant transfer of OH stretching quanta between water molecules, typically occurring on a 100 fs time scale. In DNA oligomers, femtosecond nonlinear vibrational spectroscopy resolves NH and OH stretching bands in the highly congested infrared spectra of these molecules, which contain alternating adenine-thymine pairs. Studies at different levels of hydration reveal the spectral signatures of water molecules directly interacting with the phosphate groups of DNA and of a second water species forming a fluctuating environment around the DNA oligomers. We expect that the application of 2D infrared spectroscopy in an extended spectral range will reveal the intrinsic coupling between water and specific functional units of DNA.

Hydrogen-Bond Dynamics of Water in Ionic Solutions

We study the effects of ions on the structure and dynamics of the hydrogen bonds in liquid water. As a technique we use femtosecond two-color mid-infrared spectroscopy, since this technique allows a clear distinction of the dynamics of the first solvation (hydration) shell of water molecules from the dynamics of bulk water. We find that water molecules in the first hydration shell of the halogenic anions Cl−, Br− and I− show much slower hydrogen-bond dynamics than water molecules in the pure liquid. We also observe that the first hydration shell shows very slow collective orientational dynamics, and forms a rigid, long-living structure. Finally, we find that ions have surprisingly little effect on the hydrogen-bond dynamics of water molecules outside the first hydration shell, which implies that ions do not enhance or weaken the hydrogen-bond network of liquid water.

Influence of ions on the hydrogen-bond structure in liquid water

The orientational-correlation time of water molecules in ionic solutions has been measured with femtosecond pump–probe spectroscopy. It is found that the addition of ions has no influence on the rotational dynamics of water molecules outside the first solvation shells of the ions. This shows that the presence of ions does not lead to an enhancement or a breakdown of the hydrogen-bond network in liquid water.

Negligible effect of ions on the hydrogen-bond structure in liquid water.

The effects of ions on bulk properties of liquid water, such as viscosity, have suggested that ions alter water's hydrogen-bonding network. We measured the orientational correlation time of water molecules in Mg(ClO4)2, NaClO4, and Na2SO4 solutions by means of femtosecond pump-probe spectroscopy. The addition of ions had no influence on the rotational dynamics of water molecules outside the first solvation shells of the ions. This result shows that the presence of ions does not lead to an enhancement or a breakdown of the hydrogen-bond network in liquid water.

Topological properties of the hydrogen-bond network in liquid water

Spatial correlations extend about 10 Å in liquid water at room temperature and atmospheric pressure. We have studied these correlations from a topological point of view. Using a novel, effective hydrogen-bond definition, a graph representation of the hydrogen-bond network has been constructed from molecular configurations, generated by classical molecular dynamics simulation. We have developed a scheme to characterize the local topology in terms of different types of loops and chains of water molecules. From this, a detailed picture of the local topology emerges. A comparison with the predictions of a combinatorial model indicates how the proton ordering in SPC/E water differ from that in a random network.

Low-Density "Patches" in the Hydrogen-Bond Network of Liquid Water: Evidence from Molecular-Dynamics Computer Simulations

The connectivity properties of liquid water are studied with molecular dynamics. It is found that the hydrogen-bonded network includes tiny spatially correlated "patches" of four-bonded molecules, and that the local density near a "patch" is lower than the global density. The diameter of the finite patches increases with decreasing temperature, and is comparable to the characteristic length scale of density fluctuations recently measured by small-angle x-ray scattering.