Revati Kumar

Hydrogen bonding definitions and dynamics in liquid water

X-ray and neutron diffractions, vibrational spectroscopy, and x-ray Raman scattering and absorption experiments on water are often interpreted in terms of hydrogen bonding. To this end a number of geometric definitions of hydrogen bonding in water have been developed. While all definitions of hydrogen bonding are to some extent arbitrary, those involving one distance and one angle for a given water dimer are unnecessarily so. In this paper the authors develop a systematic procedure based on two-dimensional potentials of mean force for defining cutoffs for a given pair of distance and angular coordinates. They also develop an electronic structure-based definition of hydrogen bonding in liquid water, related to the electronic occupancy of the antibonding OH orbitals. This definition turns out to be reasonably compatible with one of the distance-angle geometric definitions. These two definitions lead to an estimate of the number of hydrogen bonds per molecule in liquid simple point charge/extended (SPC/E) water of between 3.2 and 3.4. They also used these and other hydrogen-bond definitions to examine the dynamics of local hydrogen-bond number fluctuations, finding an approximate long-time decay constant for SPC/E water of between 0.8 and 0.9 ps, which corresponds to the time scale for local structural relaxation.

Hydrogen bonding and Raman, IR, and 2D-IR spectroscopy of dilute HOD in liquid D2O

We present improvements on our previous approaches for calculating vibrational spectroscopy observables for the OH stretch region of dilute HOD in liquid D2O. These revised approaches are implemented to calculate IR and isotropic Raman spectra, using the SPC/E simulation model, and the results are in good agreement with experiment. We also calculate observables associated with three-pulse IR echoes: the peak shift and 2D-IR spectrum. The agreement with experiment for the former is improved over our previous calculations, but discrepancies between theory and experiment still exist. Using our proposed definition for hydrogen bonding in liquid water, we decompose the distribution of frequencies in the OH stretch region in terms of subensembles of HOD molecules with different local hydrogen-bonding environments. Such a decomposition allows us to make the connection with experiments and calculations on water clusters and more generally to understand the extent of the relationship between transition frequency and local structure in the liquid.

A second generation distributed point polarizable water model.

A distributed point polarizable model (DPP2) for water, with explicit terms for charge penetration, induction, and charge transfer, is introduced. The DPP2 model accurately describes the interaction energies in small and large water clusters and also gives an average internal energy per molecule and radial distribution functions of liquid water in good agreement with experiment. A key to the success of the model is its accurate description of the individual terms in the n-body expansion of the interaction energies.

Water simulation model with explicit three-molecule interactions.

Much effort has been directed at developing models for the computer simulation of liquid water. The simplest models involve effective two-molecule interactions, parametrized from experiment, for use in classical molecular dynamics simulations. These models have been very successful in describing the structure and dynamics of liquid water at room temperature and one atmosphere pressure. A completely successful model, however, should be robust enough to describe the properties of liquid water at other thermodynamic points, waters complicated phase diagram, heterogeneous situations like the liquid/vapor interface, ionic, and other aqueous solutions, and confined and biological water. In this paper, we develop a new classical simulation model with explicit three-molecule interactions. These interactions presumably make the model more robust in the senses described above, and since they are short-ranged, the model is efficient to simulate. The model is formulated as a perturbation from a classical two-molecule interaction model, where the forms of the correction to the two-molecule term and the three-molecule terms result from electronic structure calculations on dimers and trimers. The magnitudes of these perturbations, however, are determined empirically. The resulting model improves upon the well-known two-molecule interaction models for both static and dynamic properties.

Propensity of Hydrated Excess Protons and Hydroxide Anions for the Air–Water Interface

Significant effort has been undertaken to better understand the molecular details governing the propensity of ions for the air-water interface. Facilitated by computationally efficient reactive molecular dynamics simulations, new and statistically conclusive molecular-scale results on the affinity of the hydrated excess proton and hydroxide anion for the air-water interface are presented. These simulations capture the dynamic bond breaking and formation processes (charge defect delocalization) that are important for correctly describing the solvation and transport of these complex species. The excess proton is found to be attracted to the interface, which is correlated with a favorable enthalpic contribution and consistent with reducing the disruption in the hydrogen bond network caused by the ion complex. However, a recent refinement of the underlying reactive potential energy function for the hydrated excess proton shows the interfacial attraction to be weaker, albeit nonzero, a result that is consistent with the experimental surface tension measurements. The influence of a weak hydrogen bond donated from water to the protonated oxygen, recently found to play an important role in excess hydrated proton transport in bulk water, is seen to also be important for this study. In contrast, the hydroxide ion is found to be repelled from the air-water interface. This repulsion is characterized by a reduction of the energetically favorable ion-water interactions, which creates an enthalpic penalty as the ion approaches the interface. Finally, we find that the fluctuation in the coordination number around water sheds new light on the observed entropic trends for both ions.

A Visible-Light-Promoted O-Glycosylation with a Thioglycoside Donor

Visible-light irradiation of 4-p-methoxyphenyl-3-butenylthioglucoside donors in the presence of Umemotos reagent and alcohol acceptors serves as a mild approach to O-glycosylation. Visible-light photocatalysts are not required for activation, and alkyl- and arylthioglycosides not bearing the p-methoxystyrene are inert to these conditions. Experimental and computational evidence for an intervening electron donor-acceptor complex, which is necessary for reactivity, is provided. Yields with primary, secondary, and tertiary alcohol acceptors range from moderate to high. Complete β-selectivity can be attained through neighboring-group participation.

A Modified MSEVB Force Field for Protonated Water Clusters

A multistate empirical valence bond (MSEVB) model for protonated water clusters, which incorporates the TIP4P water model, is presented. This model which is designated MSEVB4P represents a significant improvement over the original model of Voth et al. (J. Phys. Chem. B 1998, 102, 5547) which was based on the TIP3P water model and a smaller improvement over the recently introduced MSEVB3 model (J. Phys. Chem. B 2008, 112, 467) which is based on the SPC/Fw (J. Chem. Phys. 2006, 124, 024503) water model.

Proton Transport under External Applied Voltage

Proton transport through an electrolyte layer between platinum electrodes under a range of applied voltages is explored using reactive molecular dynamics simulation. The proton transport process is decomposed into vehicular and Grotthuss hopping components, and the two mechanisms and their correlation are investigated as a function of applied voltage. At higher applied voltages, the effect of the hopping mechanism is much larger as compared with the vehicular mechanism. As the voltage is increased, the net correlation between the two mechanisms goes from negative to positive, and both the hopping frequency as well as the number of consecutive forward hops increases. This behavior results in a larger total diffusion constant at higher values of the voltage. The behavior of the hydrated excess proton is therefore substantially different under an applied external voltage than in the normal bulk water environment.

Classical simulations with the POLIR potential describe the vibrational spectroscopy and energetics of hydration: divalent cations, from solvation to coordination complex.

POLIR, a polarizable water potential optimized for vibrational and intermolecular spectroscopy in pure water but not optimized for solvation, is used to describe solutions of the divalent metal cations Ca(2+), Mg(2+), and Cu(2+). The spectral shifts in the O-H stretch region obtained from classical simulations are in agreement with experiment. The water-ion binding energies are dominated by classical electrostatics, even though the Cu(2+) case might be considered to involve an intermediate-strength chemical bond. Three-body energies of the ion with the first solvation shell are in agreement with ab initio calculations. Our results indicate the importance of polarization in the development of accurate, transferable, force fields and the power of classical methods when it is carefully included.

Exploring the behaviour of the hydrated excess proton at hydrophobic interfaces

The affinity of the excess proton for the aqueous solution-hydrophobic interface was examined for two specific examples, the air-water and hydrophobic wall-water cases, using a multiconfigurational molecular dynamics algorithm. The use of a reactive simulation method is important as it allows for a realistic description of the excess proton, namely, its propensity to hop between water molecules via the Grotthuss mechanism. The free energy profile reveals a minimum at these interfaces due to a favourable enthalpic term that outweighs the entropic penalty. The key factors that contribute to this enthalpic minimum were examined using a generalization of a scheme that decomposes the interaction energy into separate terms arising from various local environments [Otten et al., Proc. Natl. Acad. Sci. USA, 109, 701 (2012)] (coordination shell, bulk, and interface) and the delocalization energy (which allows the proton to hop). For both systems, it was observed that the energetic penalty for loss of coordinating water molecules as the excess proton moves toward the hydrophobic interface is more than compensated by the displacement of unfavourable interfacial water molecules. In addition, the ion becomes more delocalized, more Zundel-like, and therefore possesses a larger effective radius as it moves to the interface. The fluctuations of the instantaneous interface were reduced near the vicinity of the ion, thereby giving rise to an entropic penalty. This paper will discuss the application of energy decomposition schemes to multiconfigurational simulations and the resulting consequences realized for the excess proton at hydrophobic interfaces.