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

Structure and dynamics of water remain a challenge. Resolving the properties of hydrogen bonding lies at the heart of this puzzle. We employ ab initio Molecular Dynamics (AIMD) simulations over a wide temperature range. The total simulation time was < 2 ns. Both bulk water and water in the presence of a small hydrophobic molecule were simulated. We show that large-angle jumps and bond bifurcations are fundamental properties of water dynamics and that they are intimately coupled to both local density and hydrogen bond strength oscillations in scales from about 60 to a few hundred femtoseconds: Local density differences are the driving force for bond bifurcations and the consequent large-angle jumps. The jumps are intimately connected to the recently predicted hydrogen bond energy asymmetry. Our analysis also appears to confirm the existence of the so-called negativity track provided by the lone pairs of electrons on the oxygen atom to enable water rotation. S tatic and dynamic properties of both bulk and solvation water have garnered a lot of attention including several conflicting hypotheses. For example, the picture that water rotation can be desribed by simple Debye rotational diffusion was shown to be too simplistic by Laage and Hynes who demonstrated, using computer simulations, that instead of smooth rotation, water molecules undergo large-angle jumps 1 . This has since been confirmed by experiments 2 and independent simulations 3 . In a recent contribution, ab initio simulations of Kühne and Khaliullin 4 resolved the controversy regarding the interpretation of a series of x-ray experiments 5,6 that suggested water having anisotropic structure. Their simulations showed that the structure of water is tetrahedral but the energetics of the hydrogen bonds (HBs) are asymmetric.

Science (New York, N.Y.), 2015

Despite decades of study, the structures adopted to accommodate an excess proton in water and the mechanism by which they interconvert remain elusive. We used ultrafast two-dimensional infrared (2D IR) spectroscopy to investigate protons in aqueous hydrochloric acid solutions. By exciting O-H stretching vibrations and detecting the spectral response throughout the mid-IR region, we observed the interaction between the stretching and bending vibrations characteristic of the flanking waters of the Zundel complex, [H(H2O)2](+), at 3200 and 1760 cm(-1), respectively. From time-dependent shifts of the stretch-bend cross peak, we determined a lower limit on the lifetime of this complex of 480 femtoseconds. These results suggest a key role for the Zundel complex in aqueous proton transfer.

2013

Liquid water consists of a highly dynamic network of hydrogen bonds, which evolves on timescales ranging from tens of femtoseconds to a few picoseconds. The fast structural evolution of water&#39;s hydrogen bond network is at the heart of numerous fundamental aqueous processes, such as proton transport, solvation, the hydrophobic effect and protein folding. In this thesis, I present our efforts in understanding the dynamics governing hydrogen bond switching and vibrational energy dissipation in water, and the transport of excess protons in strong acid solutions. We use ultrafast nonlinear infrared spectroscopy to study hydrogen bond and proton transfer dynamics in water and acids since vibrational frequencies, intensities and line shapes are closely associated with chemical structure and dynamics. We employed and characterized a new source of ultrafast broadband infrared pulses that span the entire mid-infrared region from 4000 cm-1 down to hundreds of cm-I, with &lt;70 fs pulse dur...

Journal of Physics: Condensed Matter, 2005

We use femtosecond two-colour mid-infrared spectroscopy to study the dynamics of aqueous solutions of salts in HDO:D 2 O. We find that the lifetime of the O-H stretch vibration of HDO molecules in the solvation shell of the halogenic anions Cl − , Br − , and I − is much longer than the lifetime of the O-H stretch vibration of the HDO molecule in bulk D 2 O solution. This difference in lifetime allows for a clear separation of the response of the solvation shell from that of the bulk liquid. We observe that the solvating HDO molecules show much slower hydrogen-bond and reorientation dynamics than bulk water molecules. In contrast, the dynamics of the water molecules outside the first ionic hydration shell are observed to be negligibly affected by the ions.

The Journal of Physical Chemistry B, 2008

We detail and considerably extend the analysis recently presented in Science 2006, 311, 832-835 of the molecular mechanism of water reorientation based on molecular dynamics simulations and the analytic framework of the extended jump model (EJM). The water reorientation is shown to occur through largeamplitude angular jumps due to the exchange of hydrogen (H)-bond acceptors, with a minor contribution from the diffusive H-bond frame reorientation between these exchanges. The robust character of this mechanism with respect to different water models is discussed. We fully characterize these jump events, including the distributions of trajectories around the average path. The average path values and the distributions of the jump time and the jump amplitude, the two key parameters in the Ivanov jump model component of the EJM, are determined. We also discuss the possibility of selectively exciting water molecules close to the jump event, of interest for ultrafast infrared experiments. In addition to a comparison of predicted reorientation times with experimental results, the reorientation time temperature dependence is discussed. A detailed description of the pathway free energetics for the water reorientation is presented; this is used to identify the jump rate-limiting step as the translational motion in which the initial H-bond of the reorientating water is elongated and the new H-bond acceptor water approaches.

The molecular reorientation of liquid water is key to the hydration and stabilization of molecules and ions in aqueous solution. A powerful technique to study this reorientation is to measure the time-dependent anisotropy of the excitation of the O-H/O-D stretch vibration of HDO dissolved in D 2 O/H 2 O using femtosecond midinfrared laser pulses. In this paper, we present and discuss experiments in which this technique is used to study the correlation between the molecular reorientation of the water molecules and the strength of the hydrogen-bond interactions. On short time scales (<200 fs), it was found that the anisotropy shows a partial decay due to librational motions of the water molecules that keep the hydrogen bond intact. On longer time scale (>200 fs), the anisotropy shows a complete decay with an average time constant of 2.5 ps. From the frequency dependence of the anisotropy dynamics, it follows that a subensemble of the water molecules shows a fast reorientation that is accompanied by a large change of the vibrational frequency. This finding agrees with the molecular jumping mechanism for the reorientation of liquid water that has recently been proposed by Laage and Hynes.

We study hydrogen-bond dynamics in liquid water at low temperatures using molecular dynamics simulations, and find results supporting the hypothesized continuity of dynamic functions between the liquid and glassy states of water. We find that average bond lifetime ͑ϳ1 ps͒ has Arrhenius temperature dependence. We also calculate the bond correlation function decay time ͑ϳ1 ns͒ and find powerlaw behavior consistent with the predictions of the mode-coupling theory, suggesting that the slow dynamics of hydrogen bonds can be explained in the same framework as standard transport quantities.

Journal of Physics: Condensed Matter, 2002

The hydrogen bond (H-bond) in liquid water holds the key to its peculiar behavior, with implications for chemical, biological and geological processes. In liquid water, the dynamical motion of atoms at the picosecond time-scale causes the H-bonds to break and reform resulting in a statistical distribution of different coordinations for the water molecules. Water molecules in liquid and solid phases exhibit two types of O-H interactions: strong covalent O-H bonds within the water molecule, and relatively weak H-bonds between the molecules. In ice, each water molecule is tetrahedrally coordinated by four neighboring waters through H-bonds (2 H-bonds to the oxygen atom, and one to each hydrogen). Although liquid water primarily exhibits the same coordination environment, experimental (vibrational spectroscopy, neutron and X-ray diffraction) and theoretical (molecular dynamics) studies have predicted that liquid water should contain a fraction of water molecules with broken H-bonds.

Science, 2004

Proceedings of The National Academy of Sciences, 2005

The unique chemical and physical properties of liquid water are a direct result of its highly directional hydrogen-bond (HB) network structure and associated dynamics. However, despite intense experimental and theoretical scrutiny spanning more than four decades, a coherent description of this HB network remains elusive. The essential question of whether continuum or multicomponent (''intact,'' ''broken bond,'' etc.) models best describe the HB interactions in liquid water has engendered particularly intense discussion. Most notably, the temperature dependence of water's Raman spectrum has long been considered to be among the strongest evidence for a multicomponent distribution. Using a combined experimental and theoretical approach, we show here that many of the features of the Raman spectrum that are considered to be hallmarks of a multistate system, including the asymmetric band profile, the isosbestic (temperature invariant) point, and van't Hoff behavior, actually result from a continuous distribution. Furthermore, the excellent agreement between our newly remeasured Raman spectra and our model system further supports the locally tetrahedral description of liquid water, which has recently been called into question [Wernet, P., et al. (2004) Science 304, 995-999].