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.