The reaction mechanism for acetic acid dissociation in aqueous solution has been investigated by combining the metadynamics method with transition path sampling (TPS). By using collective variables that describe not only the deprotonation of the acid but also the solvation state of the hydronium ion and its distance from the acetate, a reactive trajectory in which stable separated ions were produced was obtained. More transition trajectories were sampled by using the TPS technique, taking the metadynamics trajectory as the initial trajectory. Two different dissociation reaction pathways were determined, one driven by the breaking of an H-bond formed by the water molecule in contact with the hydroxyl and involving the formation of a metastable contact ion pair and the other characterized by a direct transfer of the proton to the solution through an unstable Zundel-ion-like intermediate.

The hydroxide ion (OH-) has an unusually high mobility in water, comparable to that of the proton. However, a consensus view of the OH- mobility mechanism and its solvation structure has yet to emerge. In addition, X-ray and spectroscopic experiments reveal significant changes in the structural and dynamical properties of water in the presence of OH-. To gain insight into these questions, we have carried out Car−Parrinello molecular dynamics (CPMD) calculations for aqueous NaOH and KOH solutions under ambient conditions over a wide range of concentrations. These simulations are able to reproduce many puzzling phenomena, in particular, the loss of tetrahedral coordination of water (interpreted from a recent neutron diffraction with isotopic substitution experiment) and the appearance of new spectroscopic features at high concentrations. Furthermore, it is demonstrated that the observed behavior is a result of the formation of a variety of compact hydroxide−water complexes. The distribution of these complexes is dependent upon the concentration and the counterion. The present results reconcile conflicting structural interpretations from previous experimental and theoretical studies on hydroxide solutions. Analysis of the CPMD trajectories supports the view that the transport mechanism of the hydroxide ion is distinct from that of the proton.

Car-Parrinello molecular dynamics simulations have been carried out for aqueous NaOH and KOH solutions under ambient conditions over a wide range of concentrations. From these simulations, we have observed a continuous change of the water structure with added hydroxide, characterized by a significant shift of the second peak of the OO radial distribution functions to shorter distances. At the highest concentration investigated, the normal tetrahedral coordination of pure water is completely missing, a result that is consistent with a recent neutron diffraction experiment. The added hydroxide also gives rise to some unique spectroscopic features, including a "free" O-H stretch, a broadening of the normal water OH stretching band, and a large blue shift of both the librational band and the low-frequency translation. These results are in good agreement with the experimental data. Finally, it was demonstrated that the structural and dynamical behavior is inextricably linked to the formation of compact hydroxide-water complexes.

The adsorption of water on the MgO͑001͒ surface is studied by using density-functional theory calculations within the generalized gradient approximation. Our calculations show that coupled three and four water molecules are partly dissociated, indicating that the intermolecular hydrogen bonding plays an important role in water dissociation on MgO͑001͒. Especially, four water molecules are found to be significantly stabilized due to the increase in the number of the intermolecular hydrogen bonds. This hydrogen-bonding unit can explain experimental observations of the c(4ϫ2) phase and its transition to the p(3ϫ2) phase composed of three water molecules.

We have studied the structure and energetics of adsorbed water on the NaCl(001) surface using densityfunctional calculations within the generalized gradient approximation. We predict a new adsorption structure for the c͑4 ϫ 2͒ water bilayer which is energetically favored over the previous puckered hexagonal c͑4 ϫ 2͒ structure. Our calculations show that the 1 ϫ 1 monolayer structure (wherein every water molecule binds to each surface cation) is metastable, thereby suggesting that the 1 ϫ 1 structure would be transformed to the more stable c͑4 ϫ 2͒ structure which has an increased H-bond interactions between water molecules.

Ab initio calculations have been employed to investigate the peculiar change in magnetic property (from diamagnetic to paramagnetic) of the dianionic C60-dimer phase in a rapidly cooled AC60 samples ( A: alkali metal). We first note that the triplet state of (C60)-22 which was never considered previously is nearly degenerate with the singlet state, and the transition barrier between the two states is reasonably small. This explains the susceptibility increase with an increase in temperature and the magnetic phase transition in the process of the dimer to monomer phase transition.

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Theoretical investigation of the potential energy surface of the van der Waals complex CH 4 -N 2