Hydrogen Bond Exchange Research Articles

Communication: On the origin of the non-Arrhenius behavior in water reorientation dynamics

We combine molecular dynamics simulations and analytic modeling to determine the origin of the non-Arrhenius temperature dependence of liquid water's reorientation and hydrogen-bond dynamics between 235 K and 350 K. We present a quantitative model connecting hydrogen-bond exchange dynamics to local structural fluctuations, measured by the asphericity of Voronoi cells associated with each water molecule. For a fixed local structure the regular Arrhenius behavior is recovered, and the global anomalous temperature dependence is demonstrated to essentially result from a continuous shift in the unimodal structure distribution upon cooling. The non-Arrhenius behavior can thus be explained without invoking an equilibrium between distinct structures. In addition, the large width of the homogeneous structural distribution is shown to cause a growing dynamical heterogeneity and a non-exponential relaxation at low temperature.

Aqueous Solvation of Methane from First Principles

Structural, dynamical, bonding, and electronic properties of water molecules around a soluted methane molecule are studied from first principles. The results are compatible with experiments and qualitatively support the conclusions of recent classical molecular dynamics simulations concerning the controversial issue on the presence of "immobilized" water molecules around hydrophobic groups: the hydrophobic solute slightly reduces (by a less than 2 factor) the mobility of many surrounding water molecules rather than immobilizing just the few ones which are closest to methane, similarly to what was obtained by previous first-principles simulations of soluted methanol. Moreover, the rotational slowing down is compatible with the one predicted on the basis of the excluded volume fraction, which leads to a slower hydrogen bond exchange rate. The analysis of simulations performed at different temperatures suggests that the target temperature of the soluted system must be carefully chosen, in order to avoid artificial slowing-down effects. By generating maximally localized Wannier functions, a detailed description of the polarization effects in both solute and solvent molecules is obtained, which better characterizes the solvation process.

Water Jump Reorientation: From Theoretical Prediction to Experimental Observation

Liquid water is remarkably labile in reorganizing its hydrogen-bond (HB) network through the breaking and forming of HBs. This rapid restructuring, which occurs on the picosecond time scale, is critical not only for many of the pure liquid's special features but also for a range of aqueous media phenomena, including chemical reactions and protein activity. An essential part of the HB network reorganization is water molecule reorientation, which has long been described as Debye rotational diffusion characterized by very small angular displacements. Recent theoretical work, however, has presented a starkly contrasting picture: a sudden, large-amplitude jump mechanism, in which the reorienting water molecule rapidly exchanges HB partners in what amounts to an activated chemical reaction. In this Account, we first briefly review the jump mechanism and then discuss how it is supported by a series of experiments. These studies range from indirect indications to direct characterization of the jumps through pioneering two-dimensional infrared spectroscopy (2D-IR), the power of which accords it a special focus here. The scenarios in which experimental signatures of the jump mechanism are sought increase in complexity throughout the Account, beginning with pure water. Here 2D-IR in combination with theory can give a glimpse of the jumps, but the tell-tale markers are not pronounced. A more fruitful arena is provided by aqueous ionic solutions. The difference between water-water and water-anion HB strengths provides the experimental handle of differing OH stretch frequencies; in favorable cases, the kinetic exchange of a water between these two sites can be monitored. Sole observation of this exchange, however, is insufficient to establish the jump mechanism. Fortunately, 2D-IR with polarized pulses has demonstrated that HB exchange is accompanied by significant angular displacement, with an estimated jump angle similar to theoretical estimates. The Janus-like character of amphiphilic solutes, with their hydrophobic and hydrophilic faces, presents a special challenge for theory and experiment. Here a consensus on the 2D-IR interpretation has not yet been achieved; this lack of accord impedes the understanding of, for example, biochemical solutes and interfaces. We argue that the influence of hydrophobic groups on water jumps is only modest and well accounted for by an excluded volume effect in the HB exchange process. Conversely, hydrophilic groups have an important influence when their HB strength with water differs significantly from that of the water-water HB. The power of 2D-IR is argued to be accompanied by subtleties that can lead to just the opposite and, in our view, erroneous conclusion. We close with a prediction that a hydrophobic surface offers an arena in which the dynamics of "dangling" water OHs, bereft of a HB, could provide a 2D-IR confirmation of water jumps.

Low-Temperature NMR Studies on Inosine Wobble Base Pairs

Base pairs formed by the inosine nucleoside (I) play an important role in many physiological processes as well as in various DNA technologies. Relative stabilities and favored base pair geometries of free inosine wobble base pairs in aprotic solvents have been determined through (1)H NMR measurements at room temperature and at very low temperatures in a freonic solvent. As indicated by its significantly deshielded imino proton, the Watson-Crick-type I·C base pair forms a remarkably strong NHN hydrogen bond. For the thermodynamically less stable I·A wobble base pair, two configurations of similar population coexist at 133 K in the slow hydrogen bond exchange regime, namely a Watson-Crick(I)-Watson-Crick(A) geometry and a Watson-Crick(I)-Hoogsteen(A) geometry. I·U base pairs are stabilized by two rather weak hydrogen bonds and are significantly disfavored over inosine self-associates in a low-temperature Freon solution.

Disaccharide Topology Induces Slowdown in Local Water Dynamics

Molecular level insight into water structure and structural dynamics near proteins, lipids, and nucleic acids is critical to the quantitative understanding of many biophysical processes. Unfortunately, understanding hydration and hydration dynamics around such large molecules is challenging because of the necessity of deconvoluting the effects of topography and chemical heterogeneity. Here we study, via classical all-atom simulation, the water structure and structural dynamics around two biologically relevant solutes large enough to have significant chemical and topological heterogeneity but small enough to be computationally tractable: the disaccharides kojibiose and trehalose. We find both molecules to be strongly amphiphilic (as quantified from normalized local density fluctuations) and to induce nonuniform local slowdown in water translational and rotational motions. Detailed analysis of the rotational slowdown shows that, while the rotational mechanism is similar to that previously identified in other aqueous systems by Laage, Hynes, and coworkers, two novel characteristics are observed: broadening of the transition state during hydrogen bond exchange (water rotation) and a subpopulation of water for which rotation is slowed because of hindered access of the new accepting water molecule to the transition state. Both characteristics are expected to be generic features of water rotation around larger biomolecules and, taken together, emphasize the difficulty in transferring insight into water rotation around small molecules to much larger amphiphilic solutes.

How Biomolecules Influence Water Structure and Dynamics

Interactions between solutes and water impact both water structure and structural dynamics as well as solute properties (e.g. conformational fluctuations of proteins). To understand these interactions we investigate water near disaccharides using classical atomistic molecular dynamics simulations. Disaccharides show topological and chemical complexity characteristic of larger biomolecules but are sufficiently small to permit detailed study. We observe that increases in hydrophobicity precisely map slow down in water translation and rotation of local water populations. In line with recent studies of proteins, we find that chemically similar functional groups may interact differently with water depending on neighboring functional groups.To explain these observations we examine the mechanism of hydrogen bond exchange for waters hydrogen bonded to other waters but within the sugar first solvation shell, as well as waters hydrogen bonded to the sugar. Recent work showed that water in bulk rotates through large angular jumps that pass through bifurcated hydrogen bond intermediates and that rotation rates can be rationalized through transition state theory. Previous reports found that the rotational slow down of water near small solutes can be predicted from changes in the accessible transition state volume or the enthalpy of the hydrogen bonds. For our larger solutes we find that accounting for the transition state volume alone overestimates water rotational slow down. Differences in hydrogen bond enthalpy are also insufficient to predict rotational slowdown. Water slowdown can only be understood by additionally accounting for subtle changes in the free energy landscape associated with water rotation - reduction in the number of available reactant states and broadening of the transition state barriers. The presence of solutes of even moderate size thus affects water dynamics in ways difficult to predict using simple scaling considerations from bulk, making water response system dependent.

Topological modelling of the hydrogen bond network of water cluster and proton hopping in a hydrated polyelectrolyte membrane

The water cluster is characteristically constructed through the hydrogen bond formation in various types of aqueous solutions, especially in the pore of nano-materials such as polyelectrolyte membranes. The proton conduction in such a water cluster depends on the topology of the hydrogen bond network, since the movement of the proton is mainly driven by hydrogen bond exchange in the water cluster. The first-principle electronic state calculation should be straightforwardly adopted in order to estimate the proton conductivity because the change in hydrogen bond formation is associated with the change in the electronic states of the cluster. Although the first-principle calculations of proton conductivity are basically available to estimate the rate of proton hopping in the water clusters, it would be extremely time-consuming to simulate the dynamic structure in an inhomogeneous morphology of nanometre length scale such as in the polyelectrolyte membrane based on the electronic structure theory calculations. Here, the characterisation of the hydrogen bond network via a dynamically directed graph was evaluated to calculate the dynamical properties in the global structure of nanometre length scale inhomogeneous morphologies. The static and dynamic properties of the network structure were analysed by following the treatment of the graph theory. The mean rate of Hamming displacement (MHD) was defined to quantitatively express the dynamics of the hydrogen bond network. The average of the differences between the Hamming distances at different time steps, as the definition of the MHD, showed that the relaxation of the hydrogen bond network in polyelectrolyte membranes was slower than in the pure water because of the stronger clustering in a local area due to the mesoscopic morphology or confinement effect in nano-pores. Furthermore, proton hopping was modelled as a random walk on the dynamical hydrogen bond network, and then the diffusion coefficients of proton in the Nafion membranes were estimated as comparable results with the previous literatures.

Dynamics of the Water Hydrogen Bond Network at Ionic, Nonionic, and Hydrophobic Interfaces in Nanopores and Reverse Micelles

The effects of water confinement on hydrogen bond dynamics and hydrogen bond exchange have been analyzed by molecular dynamics simulations for a series of different sizes of spherical nanopores of ionic, nonionic, and hydrophobic interfaces. We have calculated translational diffusion residence times, orientational decay time constants, the infrared spectra, correlation functions describing the hydrogen bond network, the hydrogen bond exchange time and rate constant, and ensemble averages of the hydrogen bond exchange reaction coordinate. We focus on the interfacial layer and bulklike interior of these small water containing nanostructures. Our results indicate a universal slowdown in rotational and hydrogen bond dynamics at the interface relative to bulk water. The interiors of nanopores with highly charged interfaces undergo qualitatively different dynamics than those in other nanopores. The rotational jump hydrogen bond exchange mechanism is shown to be robust and universal, even for this variety of interfaces. The implications of these results are discussed in terms of the role of confinement vs interface structure on water dynamics in nanopores.

Geometries and Tautomerism of OHN Hydrogen Bonds in Aprotic Solution Probed by H/D Isotope Effects on 13C NMR Chemical Shifts

The (1)H and (13)C NMR spectra of 17 OHN hydrogen-bonded complexes formed by CH(3)(13)COOH(D) with 14 substituted pyridines, 2 amines, and N-methylimidazole have been measured in the temperature region between 110 and 150 K using CDF(3)/CDF(2)Cl mixture as solvent. The slow proton and hydrogen bond exchange regime was reached, and the H/D isotope effects on the (13)C chemical shifts of the carboxyl group were measured. In combination with the analysis of the corresponding (1)H chemical shifts, it was possible to distinguish between OHN hydrogen bonds exhibiting a single proton position and those exhibiting a fast proton tautomerism between molecular and zwitterionic forms. Using H-bond correlations, we relate the H/D isotope effects on the (13)C chemical shifts of the carboxyl group with the OHN hydrogen bond geometries.

Large Angular Jump Mechanism Observed for Hydrogen Bond Exchange in Aqueous Perchlorate Solution

The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the H-bond exchange mechanism in aqueous NaClO4 solution. The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic model, we extracted an average jump angle of 49 +/- 4 degrees, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO4 solution.

Hydrolytic Weakening of Quartz and Other Silicates*

Rocks at high pressure and temperature become weak when exposed to an aqueous environment. Four different weakening processes have been found: (1) Increase in pore pressure reduces the effective confining pressure, thus negating the strengthening effect of confining pressure (Terzaghi). (2) Penetration of water into the intergranular boundaries reduces the cohesive strength. This is believed to have occurred in the dehydration weakening found in gypsum (Heard & Rubey) and in serpentinite (Raleigh & Paterson). (3) Water promotes recrystallization, greatly reducing the strength at low strain-rates at temperatures above the critical temperature for recrystallization. (4) Water permeates most silicate crystals so far tested and causes hydrolytic weakening of the component crystals of rocks. The most revealing experiments on hydrolytic weakening have been those on single crystals of synthetic quartz with varying water content (Griggs & Blacic). The water content inferred from 3 micron infra-red absorption varies from 0·0015 to 0·13 wt% (100–9000 H/106 Si) in the nine crystals tested. All crystals became weak at a critical temperature dependent on the water content, varying from 380°C at 0·13% to 1070 °C at 0·0015%. The weakening process is thermally activated, is reversible, and is rate dependent. The mechanism of deformation is intracrystalline glide. MLaren has shown by transmission electron microscopy that deformed weakened synthetic quartz contains dislocation densities similar to those found in strong dry quartz deformed in the ductile regime. Annealing of water-weakened quartz removes most of the dislocations and causes the formation of bubbles. These are believed to be water bubbles formed by the reaction Si(OH)4→ SiO2+2H2O, the water migrating through the lattice. The Frank-Griggs hypothesis of hydrolytic weakening is that easy slip occurs only when Si-O-Si bridges adjacent to a dislocation are hydrolyzed by the migration of water. No Si-O bonds need be broken, for dislocation motion can occur by hydrogen bond exchange. All the evidence in hand suggests that hydrolytic weakening is general in silicates containing very small amounts of water. This phenomenon is expected to be important in the mantle, leading to a rate-dependent strength.

Water Hydrogen-Bond Dynamics around Amino Acids: The Key Role of Hydrophilic Hydrogen-Bond Acceptor Groups

Water hydrogen-bond (HB) dynamics around amino acids in dilute aqueous solution is investigated through molecular dynamics simulations and analytic modeling. We especially highlight the critical role played by hydrophilic HB acceptors: the strength of the HB formed with water has a pronounced effect on the HB dynamics, in accord with several experimental observations. In contrast, we evidence that hydrophilic HB donors induce a moderate slowdown in the water HB exchange dynamics due to an excluded volume effect, similar to that of hydrophobic groups. We present an analytic model which rationalizes the effect of all examined amino acid sites on the HB dynamics and whose predictions are in excellent agreement with the numerical simulations. This model provides the acceleration or retardation in the HB exchange time with respect to the bulk through the combination of the solute excluded volume factor with the solute-water HB strength factor, both referring to the HB exchange transition state.

14N NQR and proton NMR study of ferroelectric phase transition and proton exchange in organic ferroelectric (H2-TPPZ)(Hca)2

The complete (14)N nuclear quadrupole resonance spectrum has been measured in ferroelectric (H(2)-TPPZ)(Hca)(2) using nuclear quadrupole double resonance. The quadrupole coupling tensors are assigned to various nitrogen positions in the crystal structure. Two types of asymmetric N-H(+)...N hydrogen bonds are observed in the ferroelectric phase. A slow dynamics influencing the (14)N NQR spectrum and relaxation has been observed in the paraelectric phase. The analysis of the (14)N NQR spectra in the paraelectric phase shows that above T(c) each hydrogen bond exchanges between the two types observed in the ferroelectric phase. The change of the type of hydrogen bond is associated with the transfer of protons within the bond.

Structural Rearrangements in Water Viewed Through Two-Dimensional Infrared Spectroscopy

Compared with other molecular liquids, water is highly structured because of its ability to form up to four hydrogen bonds, resulting in a tetrahedral network of molecules. However, this underlying intermolecular structure is constantly in motion, exhibiting large fluctuations and reorganizations on time scales from femtoseconds to picoseconds. These motions allow water to play a key role in a number of chemical and biological processes. By exploiting the fact that the OH stretching frequency of dilute HOD in liquid D(2)O is highly dependent upon the configuration of the neighbor nearest to the proton, researchers have been able to track water's time-dependent structure using two-dimensional infrared (2D IR) spectroscopy, which tags molecules at an initial frequency and then watches as that frequency evolves with respect to time. Recent advances in molecular dynamics simulation techniques allow for the calculation of 2D IR spectra, providing an atomistic interpretation tool of 2D IR spectra in terms of the underlying dynamics of the liquid. In this Account, we review recent ultrafast 2D IR studies at MIT that provide new information on the mechanism of hydrogen-bond rearrangements in liquid water. The 2D IR spectra of the OH stretching vibration of HOD in D(2)O appear highly asymmetric. In the frequency range indicative of hydrogen-bonded molecules (<3300 cm(-1)), the 2D spectra remain fairly compact. By contrast, in the frequency range in which molecules having weak or broken hydrogen bonds absorb (>3500 cm(-1)), the 2D spectra broaden over a time scale of approximately 60 fs, consistent with librations (hindered rotations) of water molecules. This broadening indicates that molecules forming weak or broken hydrogen bonds are unstable and reorient rapidly to return to a hydrogen-bonded configuration. These conclusions are supported by the results of molecular dynamics simulations, which suggest that water molecules undergo a large-angle reorientation during the course of hydrogen-bond exchange. The transition state for hydrogen-bond rearrangements is found to resemble a bifurcated hydrogen bond. Roughly half of the hydrogen-bond exchange events in the simulation are found to involve the insertion of a water molecule across a hydrogen bond, suggesting that hydrogen-bond exchange in water involves the correlated motion of water molecules as far away as the second solvation shell. The combination of ultrafast 2D IR spectroscopy with simulation-based modeling is leading to self-consistent descriptions of the underlying dynamics in liquid water. Moreover, these results also demonstrate a more general, unique characteristic of the spectroscopy: if a spectral signature of the transition state exists, then 2D IR can effectively serve as a transition-state spectroscopy.

OHO Hydrogen Bond Geometries and NMR Chemical Shifts: From Equilibrium Structures to Geometric H/D Isotope Effects, with Applications for Water, Protonated Water, and Compressed Ice

AbstractHydrogen bond geometries and 1H NMR chemical shifts of OHO hydrogen‐bonded systems have been analyzed using an improved valence bond order model. This model predicts that the heavy atom hydrogen bond coordinate q2 = r1 + r2 is a function of the proton coordinate q1 = (r1 ‐ r2), where r1 and r2 represent the OH and the HO distances.In the first part, it is shown that this correlation reproduces published equilibrium geometries of the Zundel cation H5O2+ as well as those of water clusters in the gas phase and embedded in the fullerene C180. Using the example of the water hexamer, it is shown that changing the level of calculation shifts the calculated geometries along the correlation curve, but not away from the curve. In order to take quantum zero‐point vibrational effects (QZPVE) into account, an empirical correction is proposed. It is shown that this correction properly describes the calculated classical and quantum hydrogen bond geometries of compressed ice as well as calculated geometric H/D isotope effects. The improved valence bond order model is used to analyze a large number of OHO hydrogen bond geometries contained in the Cambridge Structural Database.In the second part, a relation between the geometries and the 1H NMR chemical shieldings of OHO hydrogen bonded systems is established using the valence bond order model. GIAO calculations of the isolated symmetric Zundel cation where H is located in the hydrogen bond center show only a small dependence of the chemical shifts on the O…O distance. This result is rationalized in terms of neighbor group effects and deshielding in the naked proton. The consequence is that the 1H NMR chemical shifts are not much affected by QZPVE. Calculations on water clusters indicate that the influence of the chemical environment of the OHO hydrogen bonds on their 1H NMR chemical shifts is smaller for the strong hydrogen bond regime but large for the weak hydrogen bond regime. A simple chemical shift vs. q1 relation is then used to calculate the average chemical shifts of water clusters in the regime of fast hydrogen bond exchange between hydrogen bonded and free OH groups. It is shown that average chemical shifts of about 6 ppm are possible as the clusters considered exhibit a broad distribution of stronger and weaker hydrogen bonds. The implications for water in organic solvents and for liquid water are discussed, based on published data on the 1H chemical shift distribution in the latter.

How Vibrationally Assisted Tunneling with STM Affects the Motions and Reactions of Single Adsorbates

A mechanism of vibrationally assisted tunneling is proposed, which combines the atomic tunneling from the vibrational states with the vibrational ladder climbing, to explain the recent experiments on adsorbate motions induced by inelastic tunneling currents with a scanning tunneling microscope. Particularly, the hydrogen-bond exchange reaction within a single-water-heavy-water dimer on a Cu(110) surface, and Co adatom hopping on a Cu (111) surface, are analyzed. It is found that the vibrationally assisted tunneling mechanism can play a key role in the adsorbate dynamical motion when the energy of the relevant vibrational excitation is lower than the barrier for motion or reaction.

Why Water Reorientation Slows without Iceberg Formation around Hydrophobic Solutes

The dynamics of water molecules next to hydrophobic solutes is investigated, specifically addressing the recent controversy raised by the first time-resolved observations, which concluded that some water molecules are immobilized by hydrophobic groups, in strong contrast to previous NMR conclusions. Through molecular dynamics simulations and an analytic jump reorientation model, we identify the water reorientation mechanism next to a hydrophobic solute and provide evidence that no water molecules are immobilized by hydrophobic solutes. Their moderate rotational slowdown compared to bulk water (e.g., by a factor of less than 2 at low solute concentration) is mainly due to slower hydrogen-bond exchange. The slowdown is quantitatively described by a solute excluded volume effect at the transition state for the key hydrogen-bond exchange in the reorientation mechanism. We show that this picture is consistent with both ultrafast anisotropy and NMR experimental results and that the transition state excluded volume theory yields quantitative predictions of the rotational slowdown for diverse hydrophobic solutes of varying size over a wide concentration range. We also explain why hydrophobic groups slow water reorientation less than do some hydrophilic groups.

Ion–water hydrogen-bond switching observed with 2D IR vibrational echo chemical exchange spectroscopy

The exchange of water hydroxyl hydrogen bonds between anions and water oxygens is observed directly with ultrafast 2D IR vibrational echo chemical exchange spectroscopy (CES). The OD hydroxyl stretch of dilute HOD in H(2)O in concentrated (5.5 M) aqueous solutions of sodium tetrafluoroborate (NaBF(4)) displays a spectrum with a broad water-like band (hydroxyl bound to water oxygen) and a resolved, blue shifted band (hydroxyl bound to BF(4)(-)). At short time (200 fs), the 2D IR vibrational echo spectrum has 4 peaks, 2 on the diagonal and 2 off-diagonal. The 2 diagonal peaks are the 0-1 transitions of the water-like band and the hydroxyl-anion band. Vibrational echo emissions at the 1-2 transition frequencies give rise to 2 off-diagonal peaks. On a picosecond time scale, additional off-diagonal peaks grow in. These new peaks arise from chemical exchange between water hydroxyls bound to anions and hydroxyls bound to water oxygens. The growth of the chemical exchange peaks yields the time dependence of anion-water hydroxyl hydrogen bond switching under thermal equilibrium conditions as T(aw) = 7 +/- 1 ps. Pump-probe measurements of the orientational relaxation rates and vibrational lifetimes are used in the CES data analysis. The pump-probe measurements are shown to have the correct functional form for a system undergoing exchange.

Direct Observation of Hydrogen-Bond Exchange in Small Water Clusters

The authors studied the structure and dynamics of individual water dimer and trimer on Cu(110) by using a scanning tunneling microscope (STM) and density functional theory (DFT). The dimer consists of hydrogen-bond donor and acceptor molecules, and dynamically rearranges the hydrogen bond resulting in the interchange of their roles. The large isotope effect (∼60) was founded in the interchange rate between (H2O)2 and (D2O)2, suggesting that the process involves quantum tunneling. The interchange motion was enhanced upon the excitation of an intermolecular mode that correlates with the reaction coordinate. In contrast, the water trimer was imaged as a static triangular shape. The calculation revealed that it forms a hydrogen-bond chain along the atomic row.

Direct Observation of Hydrogen-Bond Exchange within a Single Water Dimer

The dynamics of water dimers was investigated at the single-molecule level by using a scanning tunneling microscope. The two molecules in a water dimer, bound on a Cu(110) surface at 6 K, were observed to exchange their roles as hydrogen-bond donor and acceptor via hydrogen-bond rearrangement. The interchange rate is approximately 60 times higher for (H2O)2 than for (D2O)2, suggesting that quantum tunneling is involved in the process. The interchange rate is enhanced upon excitation of the intermolecular mode that correlates with the reaction coordinate.