Failure of molecular dynamics to provide appropriate structures for quantum mechanical description of the aqueous chloride ion charge-transfer-to-solvent ultraviolet spectrum.

The lowest band in the charge-transfer-to-solvent ultraviolet absorption spectrum of aqueous chloride ion is studied by experiment and computation. Interestingly, the experiments indicate that at concentrations up to at least 0.25 M, where calculations indicate ion pairing to be significant, there is no notable effect of ionic strength on the spectrum. The experimental spectra are fitted to aid comparison with computations. Classical molecular dynamic simulations are carried out on dilute aqueous Cl-, Na+, and NaCl, producing radial distribution functions in reasonable agreement with experiment and, for NaCl, clear evidence of ion pairing. Clusters are extracted from the simulations for quantum mechanical excited state calculations. Accurate ab initio coupled-cluster benchmark calculations on a small number of representative clusters are carried out and used to identify and validate an efficient protocol based on time-dependent density functional theory. The latter is used to carry out quantum mechanical calculations on thousands of clusters. The resulting computed spectrum is in excellent agreement with experiment for the peak position, with little influence from ion pairing, but is in qualitative disagreement on the width, being only about half as wide. It is concluded that simulation by classical molecular dynamics fails to provide an adequate variety of structures to explain the experimental CTTS spectrum of aqueous Cl-.

Vacuum ultraviolet spectroscopy of the lowest-lying electronic state in subcritical and supercritical water.

The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties. Here we report vacuum ultraviolet absorption spectra for the lowest-lying electronic state of subcritical and supercritical water. For subcritical water, the spectrum redshifts considerably with increasing temperature, demonstrating the gradual breakdown of the hydrogen-bond network. Tuning the density at 381 °C gives insight into the extent of hydrogen bonding in supercritical water. The known gas-phase spectrum, including its vibronic structure, is duplicated in the low-density limit. With increasing density, the spectrum blueshifts and the vibronic structure is quenched as the water monomer becomes electronically perturbed. Fits to the supercritical water spectra demonstrate consistency with dimer/trimer fractions calculated from the water virial equation of state and equilibrium constants. Using the known water dimer interaction potential, we estimate the critical distance between molecules (ca. 4.5 Å) needed to explain the vibronic structure quenching.

Hemibonding between Water Cation and Water.

The hemibonding interaction in the water dimer cation is studied using coupled cluster electronic structure methods. The hemibonded dimer cation geometry is a local minimum structure characterized by the two participating monomers having both a very short separation and a near parallel relative orientation. It is shown that the vertically ionized dimer at its optimum neutral geometry can convert to the hemibonded dimer cation structure with essentially no energetic hindrance. Direct conversion to the hemibonded structure is therefore an energetically facile alternative to the minimum energy path that connects the vertically ionized neutral water dimer to the global minimum proton-transferred structure. A substantial barrier must be surmounted to convert the hemibonded dimer cation to the proton-transferred structure. The optical absorption spectrum of the hemibonded dimer cation is characterized by three excited near-UV states, two of which have very large oscillator strengths. Relative resonance Raman intensities are estimated for the hemibonded dimer cation vibrational modes, finding the intermolecular stretching mode to be the most strongly enhanced when in near resonance with each of the near-UV excited states, and the anharmonicity and overtones of this mode are estimated. These results provide guidance for the possible observation of hemibonded cations in irradiated liquid water.

Composite method for implicit representation of solvent in dimethyl sulfoxide and acetonitrile.

A composite method for implicit representation of solvent previously developed to compute aqueous free energies of solvation is extended to accommodate the polar aprotic solvents dimethyl sulfoxide and acetonitrile. The method combines quantum mechanical calculation of the solute electronic structure with a modern dielectric continuum model for long-range electrostatic interactions with solvent and individual models for short-range interactions arising from dispersion, exchange, and hydrogen bonding. The few parameters involved are optimized to fit a standard data set of experimental solvation energies for neutrals and ions. Results are better than other models in the literature, with average errors for ions comparable to or smaller than the estimated experimental errors. Some circumstantial evidence is also obtained to support one of the competing extrathermodynamic arguments recently used to determine the solvation energies of the proton, which are needed to separate measurements of paired cation plus anion solvation energies into absolute single ion solvation energies in these solvents.

Monocarbon cationic cluster yields from N2/CH4 mixtures embedded in He nanodroplets and their calculated binding energies.

The formation of monocarbon cluster ions has been investigated by electron ionization mass spectrometry of cold helium nanodroplets doped with nitrogen/methane mixtures. Ion yields for two groups of clusters, CHmN2(+) or CHmN4(+), were determined for mixtures with different molecular ratios of CH4. The possible geometrical structures of these clusters were analyzed using electronic structure computations. Little correlation between the ion yields and the associated binding energies has been observed indicating that in most cases kinetic control is more important than thermodynamic control for forming the clusters.

Hydration Energy from a Composite Method for Implicit Representation of Solvent.

The CMIRS1.0 (composite method for implicit representation of solvent, Version 1.0) model is introduced for efficacious and inexpensive computation of hydration free energies. The method collects together several disparate models designed to describe short-range dispersion, exchange, and hydrogen bonding interactions as well as long-range electrostatic interactions. All the interactions are formulated as functionals of the solute charge density. The model uses only six adjustable parameters to determine the various short-range terms. In conjunction with an isodensity criterion that uses one parameter to determine the solute cavity size and shape, the model is tested on a large database of neutral and ionic solutes in water. The mean unsigned error compared to experiment is found to be as low as 0.8 kcal/mol for neutral solutes and 2.4 kcal/mol for ionic solutes, which is comparable to or better than other analogous approaches in the literature that invoke many more fitting parameters.

Hydrogen atom in water from ambient to high temperatures.

The aqueous hydrogen atom is studied with molecular dynamics simulations from ambient temperature to near the critical point. The radial distribution functions find a hydrogen atom coordination number of about 13 water molecules at 300 K to about 4 water molecules at 646 K. The radial and angular distribution functions indicate that first-shell water molecules tend to orient to maximize hydrogen bonding interactions with other water molecules. These orientational tendencies diminish with temperature. The calculated diffusion coefficient agrees very well with experimental results known near ambient temperatures. It fits a simple activation model to about 575 K, above which the diffusion becomes much faster than predicted by the fit. To temperatures of at least 500 K there is evidence for caging on a time scale of about 1 ps, but the evidence disappears at very high temperatures. Values of the aqueous hydrogen hyperfine coupling constant are obtained by averaging the results of density functional calculations on clusters extracted from the simulations. The hyperfine coupling calculations do not agree well with experiment for reasons that are not understood now, pointing to the need for further research on this problem.

New implicit solvation models for dispersion and exchange energies.

Implicit solvation models provide a very efficient means to estimate solvation energies. For example, dielectric continuum models are commonly used to obtain the long-range electrostatic interactions. These may be parametrized to also include in some average manner short-range interactions such as dispersion and exchange, but it is preferable to instead develop additional implicit models specifically designed for the short-range interactions. This work proposes new models for dispersion and exchange interactions between solute and solvent by adapting approaches previously developed for treatment of gas-phase intermolecular forces. The new models are formulated in terms of the charge densities of the solutes and use only three adjustable parameters. To illustrate the performance of the models, electronic structure calculations are reported for a large number of solutes in two nonpolar solvents where short-range interactions dominate and different balances pertain between attractive dispersion and repulsive exchange contributions. After empirical optimization of the requisite parameters, it is found that the errors compared to experimental solvation free energies are only about 0.4 kcal/mol on average, which is better than previous approaches in the literature that invoke many more parameters.

Water from ambient to supercritical conditions with the AMOEBA model.

The flexible polarizable AMOEBA force field for water is tested with classical molecular dynamics simulations from ambient up to supercritical conditions. Good results are obtained for the heat of vaporization, dielectric constant, self-diffusion constant, and radial distribution functions provided densities are fixed at the experimental values. If instead the densities are allowed to relax to those characteristic of the liquid-gas equilibrium for the model, then satisfactory results are obtained near ambient conditions, whereas at high temperatures the liquid densities are generally underestimated and the gas densities overestimated. As a consequence, the critical point of the model is reached at significantly too low temperature, although it occurs at approximately the correct density.

Insights into the ultraviolet spectrum of liquid water from model calculations: the different roles of donor and acceptor hydrogen bonds in water pentamers.

With a view toward a better understanding of changes in the peak position and shape of the first absorption band of water with condensation or temperature, results from electronic structure calculations using high level wavefunction based and time-dependent density functional methods are reported for water pentamers. Excitation energies, oscillator strengths, and redistributions of electron density are determined for the quasitetrahedral water pentamer in its C(2v) equilibrium geometry and for many pentamer configurations sampled from molecular simulation of liquid water. Excitations associated with surface molecules are removed in order to focus on those states associated with the central molecule, which are the most representative of the liquid environment. The effect of hydrogen bonding on the lowest excited state associated with the central molecule is studied by adding acceptor or donor hydrogen bonds to tetramer and trimer substructures of the C(2v) pentamer, and by sampling liquid-like configurations having increasing number of acceptor or donor hydrogen bonds of the central molecule. Our results provide clear evidence that the blueshift of excitation energies upon condensation is essentially determined by acceptor hydrogen bonds, and the magnitudes of these shifts are determined by the number of such, whereas donor hydrogen bonds do not induce significant shifts in excitation energies. This qualitatively different role of donor and acceptor hydrogen bonds is understood in terms of the different roles of the 1b(1) monomer molecular orbitals, which establishes an intimate connection between the valence hole and excitation energy shifts. Since the valence hole of the lowest excitation associated with the central molecule is found to be well localized in all liquid-like hydrogen bonding environments, with an average radius of gyration of ~1.6 Å that is much lower than the nearest neighbor O-O distance, a clear and unambiguous connection between hydrogen bonding environments and excitation energy shifts can be established. Based on these results, it is concluded that peak position of the first absorption band is mainly determined by the relative distribution of single and double acceptor hydrogen bonding environments, whereas the shape of the first absorption band is mainly determined by the relative distribution of acceptor and broken acceptor hydrogen bonding environments. The temperature dependence of the peak position and shape of the first absorption band can be readily understood in terms of changes to these relative populations.

Hemibonding between hydroxyl radical and water.

The ultraviolet absorption peak commonly used to identify OH radical in liquid water is mainly due to a charge-transfer-from-solvent transition that is prominent when OH is hemibonded, rather than more stable hydrogen bonded, to H(2)O. This work computationally characterizes the hemibonding interaction and the extent of the geometrical region over which it is significant. Hemibonding is found to be associated with an enlarged energy separation between the two lowest-lying electronic states, which are otherwise always quite close to one another. The lower state, wherein the hemibonding occurs, retains an attractive interaction energy between OH and H(2)O that can be as much as one-half as strong as the optimum hydrogen-bonding interaction, while the enlarged separation between the states is mainly due to the upper state becoming repulsive over most of the hemibonding region. Hemibonding also leads to a considerable drop in the energy and a considerable increase in the oscillator strength of the characteristic charge-transfer transition. The region of significant hemibonding is found to lie within a moderate range of O-O azimuthal angles and over quite wide ranges of O-O separation distances and hydroxyl OH tilt angles. In particular, significant hemibonding interactions can extend down to surprisingly short O-O distances, where the oscillator strength for the charge-transfer-from-solvent transition becomes very large.

Field-Extremum Model for Short-Range Contributions to Hydration Free Energy.

The performance in describing hydration free energies of a broad class of neutral, cationic, and anionic solutes is tested for the recently proposed FESR (Field-Extremum Short-Range) implicit solvation model for interactions between the solute and nearby water molecules, as taken in conjunction with the previously developed SS(V)PE (Surface and Simulation of Volume Polarization for Electrostatics) dielectric continuum model for long-range interactions with bulk water. The empirical FESR model mainly describes solute-water hydrogen bonding interactions by correlating them with the maximum and minimum values of the electric field produced by the solute at the surface of the cavity that excludes solvent. A preliminary report showed that, with only four adjustable parameters, the FESR model, in conjunction with SS(V)PE, can produce hydration energies comparable to the best analogous efforts in the literature that utilized many more parameters. Here, the performance of the FESR model is more fully documented in several respects. The dependence on the underlying quantum mechanical method used to treat the internal electronic structure of the solute is tested by comparing uncorrelated Hartree-Fock to correlated density functional calculations and by comparing a modest sized to a large basis set. The influence of cavity size is studied in connection with an isodensity contour construction of the cavity. The sensitivity of the results to the parameters in the FESR model is considered, and it is found that the dependence on the electric field strength is quite nonlinear, with an optimum exponent consistently in the range of 3 to 4. Overall, it is concluded that the FESR model shows considerable utility for improving the accuracy of implicit models of aqueous solvation.

How does dielectric solvation affect the size of an ion?

The effect of solvation by a continuum dielectric on the size of an ion is examined using electronic structure calculations. Various measures correlated with size are considered, including the root-mean-square radius of the electronic charge density, the amount of solute charge penetrating outside the cavity, the electronic radial distribution function, the nucleus-electron potential energy, and the electron-electron potential energy. Calculations are made on several representative ionic solutes, and it is found that every measure indicates that the application of a dielectric makes the cations larger and the anions smaller. These counterintuitive trends are examined, and a plausible explanation is offered for the observed behavior.

Insights into the ultraviolet spectrum of liquid water from model calculations.

With a view toward a better molecular level understanding of the effects of hydrogen bonding on the ultraviolet absorption spectrum of liquid water, benchmark electronic structure calculations using high level wave function based methods and systematically enlarged basis sets are reported for excitation energies and oscillator strengths of valence excited states in the equilibrium water monomer and dimer and in a selection of liquid-like dimer structures. Analysis of the electron density redistribution associated with the two lowest valence excitations of the water dimer shows that these are usually localized on one or the other monomer, although valence hole delocalization can occur for certain relative orientations of the water molecules. The lowest excited state is mostly associated with the hydrogen bond donor and the significantly higher energy second excited state mostly with the acceptor. The magnitude of the lowest excitation energies is strongly dependent on where the valence hole is created, and only to a lesser degree on the perturbation of the excited electron density distribution by the neighboring water molecule. These results suggest that the lowest excitation energies in clusters and liquid water can be associated with broken acceptor hydrogen bonds, which provide energetically favorable locations for the formation of a valence hole. Higher valence excited states of the dimer typically involve delocalization of the valence hole and/or delocalization of the excited electron and/or charge transfer. Two of the higher valence excited states that involve delocalized valence holes always have particularly large oscillator strengths. Due to the pervasive delocalization and charge transfer, it is suggested that most condensed phase water valence excitations intimately involve more than one water molecule and, as a consequence, will not be adequately described by models based on perturbation of free water monomer states. The benchmark calculations are further used to evaluate a series of representative semilocal, global hybrid, and range separated hybrid functionals used in efficient time-dependent density functional methods. It is shown that such an evaluation is only meaningful when comparison is made at or near the complete basis set limit of the wave function based reference method. A functional is found that quantitatively describes the two lowest excitations of water dimer and also provides a semiquantitative description of the higher energy valence excited states. This functional is recommended for use in further studies on the absorption spectrum of large water clusters and of condensed phase water.

Vertical electronic excitation with a dielectric continuum model of solvation including volume polarization. I. Theory.

A dielectric continuum model of solvation is developed for use in conjunction with electronic structure calculation on vertical electronic excitation of a solute. Particular attention is paid to volume polarization arising from quantum mechanical penetration of solute charge density outside the cavity that nominally encloses it, which affects both the fast and slow components of the dielectric response. An approximation that closely simulates volume polarization while being easier to implement in practice is also described. These approaches are compared to other related formulations found in the literature.

Vertical electronic excitation with a dielectric continuum model of solvation including volume polarization. II. Implementation and applications.

A practical implementation is described for calculation of solute vertical electronic excitation with a new dielectric continuum model of solvation. Particular attention is given to the specific aspects associated with quantum mechanical treatment of the solute, which leads to volume polarization effects arising from penetration of the solute charge density outside the cavity nominally enclosing it. Some representative computations are presented using this method and several other related methods from the literature for the lowest vertical transitions of an acetone and a water molecule in dielectric continuum models of aqueous solution. These illustrate the two possible extreme behaviors wherein the acetone transition is found to be little affected by volume polarization, while the water transition is found to be quite sensitive to volume polarization, the latter so much so that approximate treatments of volume polarization are inadequate.

Absorption spectrum of OH radical in water.

The influence of water on the ultraviolet absorption spectrum of OH radical is investigated with electronic structure calculations. One purpose of the work is to benchmark computational methods for their ability to treat this problem. That is done by applying a number of methods to characterization of the excited states of a variety of arrangements having OH interacting with one H(2)O molecule. In high-level coupled-cluster approaches, it is found that triple excitations are of considerable importance. Two promising methods based on highly efficient time-dependent density functional theory are identified that may provide qualitatively useful results, but no method is found that is both efficient and capable of providing quantitative accuracy. Another purpose of the work is to suggest a plausible interpretation of the experimental absorption spectrum of aqueous OH radical. For this purpose an accurate coupled cluster approach is applied to the various OH x H(2)O structures considered, along with a dielectric continuum representation of the further effects of additional bulk water. The valence transition localized on OH that is found at approximately 300 nm in gas is found to be considerably broadened by hydrogen bonding interactions with water. These transitions are assigned to the very broad shoulder on the experimental aqueous spectrum that extends from approximately 300 to 400 nm. The main experimental aqueous absorption band peaking at approximately 230 nm is found to arise instead mainly from rare hemibonded structures, which contribute out of proportion to their relative populations by virtue of having large oscillator strengths. The region near the experimental peak and on its blue side is primarily due to charge transfer transitions that move an electron to OH from hemibonded water, while the region on the near red side of the peak is primarily due to valence transitions localized on OH that is interacting with hemibonded water.

Probing silver nanoparticles during catalytic H2 evolution.

Employing silver nanoparticles from a recently developed synthesis [Evanoff, D. D.; Chumanov, G. J. Phys. Chem. B 2004, 108, 13948] and a well-studied probe molecule, p-aminothiophenol, we follow changes at the surface of the particles during the conditioning and eventually the catalytic production of hydrogen from water using strongly reducing radicals. Injection of electrons into the particles causes pronounced variations in the intensity of the surface enhanced Raman scattering (SERS) spectrum of the probe molecule. These spectral changes are caused by changes in the Fermi-level energy that are in turn caused by changes in the silver ion concentrations or in the pH, or by changes in electron density in the particle. This correlation highlights the effect of the chemical potential on the SERS enhancement at the end of the particles synthesis. The intensity of the SERS spectra increases in the presence of the silver ions when excitation at 514 nm is utilized. When the Ag(+) ions in the colloidal suspension are completely reduced by the radicals and the particles operate in the catalytic mode, the SERS spectrum is too weak to record, but it can reversibly be recovered upon the addition of Ag(+). The effect of pH on the SERS intensity is similar in nature to that of the silver ions but is complicated by the pKa of the aminothiol and the point of zero charge (pzc) of the particles. It is hypothesized that as the particles cross the pzc (around neutral pH) the electrostatic interaction between the protonated amine headgroup of the probe and the positively charged surface increases the density of probe molecules in the perpendicular orientation at the expense of a competing species. This conversion results in enhanced SERS signals and is observable during the preconditioning stage of the particles. Indeed, adsorption isotherms of the probe indicate the presence of two species. In analogous previous observations these two species have been attributed to perpendicular and flat adsorption orientations of the deprotonated probe molecule relative to the particle surface. However, preliminary density functional calculations on relevant prototypes raise the possibility that the two species may be the probe molecule and a cationic form produced by charge transfer in the ground state from the chemisorbed probe to the metal. These two forms of the probe have differing electronic structures and vibrational frequencies, with perhaps differing orientations relative to the surface. Whichever is the correct interpretation, a neutral molecule in a flat orientation or a radical cation, this species is easier to replace than the other in competitive adsorption by ethanethiol.

Dissociative electron attachment to the hydrogen-bound OH in water dimer through the lowest anionic Feshbach resonance.

The lowest energy Feshbach resonance state of the water dimer anion is computationally studied as the hydrogen-bonded OH moiety is stretched from its equilibrium position toward the hydrogen bond acceptor. The purpose is to treat a simple model system to gain insight into how hydrogen bonding may affect dissociative electron attachment to water in condensed phases. In the case of a water monomer anion, the analogous potential surface is known to be repulsive, leading directly to dissociation of H(-). In contrast, in the dimer anion, a barrier is found to dissociation of the hydrogen-bonded OH moiety such that the migrating hydrogen can be held near the Franck-Condon region in a quasibound vibrational state for a time long compared to the OH vibrational period. This behavior is found both for the case of an icelike dimer structure and for a substantial majority of liquidlike dimer structures. These findings raise the possibility that due to effects of hydrogen bonding, a molecule-centered anionic entity that is metastable both to electron detachment and to bond dissociation may live long enough to be considered as a species in the radiolysis of condensed water phases.

New formulation and implementation for volume polarization in dielectric continuum theory.

In the use of dielectric continuum theory to model bulk solvation effects on the electronic structure and properties of a solute, volume polarization contributions due to quantum mechanical penetration of the solute charge density outside the cavity nominally enclosing it are known to be significant. This work provides a new formulation and implementation of methods for solution of the requisite Poisson equation. In previous formulations the determination of the surface polarization contribution required evaluation of the difficult to calculate electric field generated by the volume polarization. It is shown that this problematic quantity can be eliminated in favor of other more easily evaluated quantities. That formal advance also opens the way for a more efficient apparatus to be implemented for calculation of the direct contribution of volume polarization to the solvation energy. The new formulation and its practical implementation are described, and illustrative numerical results are given for several neutral and ionic solutes to study the convergence and precision in practice.