Hydrogen bond (HB) connectivity in aqueous electrolyte solutions at ambient and supercritical conditions has been investigated by molecular dynamics techniques. Alkali metal and halides with different sizes have been considered. Modifications in the water HB architecture are more noticeable in the first ionic solvation shells and do not persist beyond the second shells. The coordination pattern is established between partners located in the first and second solvation shells. High-temperature results show dramatic reductions in the coordination number of water; at liquidlike densities the number of HBs is close to 2, while in steamlike environments water monomers are predominant. The addition of ions does not bring important modifications in the original HB structure for pure water. From the dynamical side, the lifetime of HBs shows minor modifications due to the simultaneous competing effects from a weaker HB structure combined with a slower reorientational dynamics of water induced by the Coulomb coupling with solute. At supercritical conditions, the overall dynamics of HB is roughly 1 order of magnitude faster than that at ambient conditions, regardless of the particular density considered.

We have extended the reference interaction site model (RISM)-polaron theory of Chandler er al. [J. Chem. Phys. 81, 1975 ] to treat self-trapping and localized states of excess electrons in polar fluids. The extension is based on a new closure of the RISM equation presented herein. The theory is applied to the hydrated electron employing a simple class of electron-water pseudopotentials. Included in this class are models coinciding with those already examined by others using computer simulations. In those cases, the results for both structural and energetic properties compare well with those of simulation. The work function, or equivalently, the excess chemical potential of the hydrated electron are also computed; the theoretical result agrees with experiment to about 1%. Most interesting, however, is that as the parameter characterizing the pseudopotentials is varied, a critical parameter is found where the electron behavior changes essentially discontinuously from a trapped state to a "super"trapped state. This transition may have a direct bearing on theoretical efforts to explain the properties of solvated electrons. 4444 J. Chem. Phys. 95 (6). 15

Energetics, structural features, polarity, and melting transitions in water clusters containing up to eight molecules were studied using ab initio methods and empirical force field models. Our quantum approach was based on density functional theory performed at the generalized gradient approximation level. For the specific case of ͑H 2 O͒ 6 , we selected five conformers of similar energy with different geometries and dipolar moments. For these cases, the cyclic arrangement was found to be the only nonpolar aggregate. For ͑H 2 O͒ 8 , the most stable structures corresponded to nonpolar, cubic-like, D 2d and S 4 conformers. Higher energy aggregates exhibit a large spectrum in their polarities. The static polarizability was found to be proportional to the size of the aggregates and presents a weak dependence with the number of hydrogen bonds. In order to examine the influence of thermal fluctuations on the aggregates, we have performed a series of classical molecular dynamics experiments from low temperature up to the melting transition using two different effective pseudopotentials: the TIP4P and MCY models. Minimum energy structures for both classical potentials were found to reproduce reasonably well the results obtained using ab initio methods. Isomerization and phase transitions were monitored by following changes in dipole moments, number of hydrogen bonds and Lindemann's parameter. For ͑H 2 O͒ 6 and ͑H 2 O͒ 8 , the melting transitions were found at T m Ϸ50 and 160 K, respectively; for both aggregates, we observed premelting transitions between well differentiated conformers as well.

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Dynamical aspects of the dielectric response of supercritical water following an instantaneous charge jump on an initially neutral Lennard-Jones solute are investigated using molecular dynamics. The SPC model was used to describe solvent−solvent interactions. The simulation experiments were performed over the density interval spanning from 0.3 up to 1 g cm-3 along the 645 K isotherm. Compared to room temperature results, the overall solvation process at high densities is an order of magnitude faster and becomes progressively slower as we move toward lower densities. In all cases, the nonequilibrium solvent responses present a bimodal behavior characterized by a fast inertial regime lasting a few femtoseconds followed by a much slower diffusional regime that dominates the long time behavior. This last portion of the response, which contributes to a small extent at high densities, accounts for the major contribution at lower densities. Predictions from linear response theory are quite accurate at high densities and become less adequate at lower densities. Instantaneous normal-mode analysis of the dynamics of the pure solvent and of the early stages of solvation are also performed; rotational modes provide the major contribution to the short time dynamics of the response at all densities.

A molecular-dynamics study of adiabatic proton transfer between two ions in a polar solvent is presented. The proton is treated as a quantum particle in three dimensions and the polar solvent is composed of classical rigid, dipolar molecules. The coupled Schrödinger and Newton’s equations are solved to determine the proton charge density and solvent configuration. The rate coefficient for the proton transfer is computed from correlation function expressions and corrections to transition-state theory due to recrossing of a free-energy barrier are determined. The simulation results are compared with a simple two-state model.

We present results of molecular dynamics simulations of solvation dynamics of coumarin 153 in dimethylsulfoxide ͑DMSO͒-water mixtures of different compositions (x D ϭ0.00, 0.25, 0.32, 0.50, 0.75, and 1.00͒ using an all-atom model for the solute probe. Results are reported for the global solvation responses of the simulated systems, as well as for the separate contributions from each cosolvent and the individual solute-site couplings to water and DMSO. The solvation dynamics is predominantly given by DMSO's contribution, even at low ͑25%͒ DMSO content, because of the preferential solvation of the probe. We find that the water molecules are only mildly coupled to the charge transfer in the coumarin, resulting in a small, largely diffusive, water relaxation component. Simulation results, including solvation responses, characteristic times, and Stokes shifts are compared with recent fluorescence upconversion experimental measurements showing good agreement for the relaxation but significant differences for the shifts.

The structural, energetic, and electronic properties of the TiO2−electrolyte interface in dye-sensitized solar cells is studied by molecular dynamics simulations and electronic structure calculations. The investigation enlightens the mechanisms responsible for the recombination of photoelectrons with redox species in the electrolyte (back-reaction effect), taking into account the important influence of surface defects, the underlying solvent dynamics, and the presence of pyridine additives at the interface. The free-energy barrier for the adsorption of redox species at the TiO2 surface is calculated. Electronic structure calculations of the TiO2/redox/solvent system evidence the distinct recombination mechanisms for the different redox species. The study provides a deeper insight on the molecular processes taking place at the interface and should stimulate further theoretical and experimental investigations.