Molecular dynamics (MD) simulation of the Mg/Al (3:1) layered double hydroxide (LDH), hydrotalcite (HT), containing the monocarboxylic acids formate, acetate, and propanoate as the charge balancing interlayer anions provides new molecular-scale insight into the interlayer structure, hydrogen bonding, and energetics of hydration and consequent swelling of LDH compounds containing organic molecules and biomolecules with carboxylate functional groups. As for citrate-HT (Kumar, P. P.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. B 2006, 110, 3841), the hydration energy of these systems as a function of water content has no distinct minima, indicating the absence of energetically well-defined structural states with specific water contents. The hydration energies, however, approach the energy of bulk liquid water at lower water contents than for citrate-HT, suggesting that synthesis strategies involving delamination of the hydroxide layers in water are likely to be more successful using starting compounds containing larger molecules with multiple carboxylate groups. This result is consistent with recent experimental observations of the delamination of lactate-HT (Hibino, T.; Kobayashi, W.

A comprehensive metadynamics study of the energetics, stability, conformational changes, and mechanism of dissociation of gas phase carbonic acid, H2CO3, yields significant new insight into these reactions. The equilibrium geometries, vibrational frequencies, and conformer energies calculated using the density functional theory are in good agreement with the previous theoretical predictions. At 315 K, the cis-cis conformer has a very short life time and transforms easily to the cis-trans conformer through a change in the O=C-O-H dihedral angle. The energy difference between the trans-trans and cis-trans conformers is very small (approximately 1 kcal/mol), but the trans-trans conformer is resistant to dissociation to carbon dioxide and water. The cis-trans conformer has a relatively short path for one of its hydroxyl groups to accept the proton from the other end of the molecule, resulting in a lower activation barrier for dissociation. Comparison of the free and potential energies of dissociation shows that the entropic contribution to the dissociation energy is less than 10%. The potential energy barrier for dissociation of H2CO3 to CO2 and H2O from the metadynamics calculations is 5-6 kcal/mol lower than in previous 0 K studies, possibly due to a combination of a finite temperature and more efficient sampling of the energy landscape in the metadynamics calculations. Gas phase carbonic acid dissociation is triggered by the dehydroxylation of one of the hydroxyl groups, which reorients as it approaches the proton on the other end of the molecule, thus facilitating a favorable H-O-H angle for the formation of a product H2O molecule. The major atomic reorganization of the other part of the molecule is a gradual straightening of the O=C=O bond. The metadynamics results provide a basis for future simulation of the more challenging carbonic acid-water system.

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An interatomic potential is proposed for the recently discovered family of superionic solids of the formula Na2M2TeO6, where M = Ni, Zn, Co, or Mg. Molecular dynamics simulations demonstrating the quality of the potential in reproducing various structural and transport properties of this promising class of materials is presented. The study provides fresh insights on the microscopic energetics and Na+ migration pathways. Strong ion–ion correlations, resulting in a highly cooperative conduction mechanism, emerge from the study.

A series of molecular dynamics (MD) simulations are carried out, in which the Na + content at the interlayers of Na 2 Ni 2 TeO 6 is systematically varied keeping the overall charge neutrality of the system, to identify the role of ion-ion correlation on Na + diffusion in the system. It has been observed that interlayers having about 20% lower concentration of Na + facilitate the highest conductivity that is an order of magnitude higher than those having normal Na + concentration. The simulations predict a gradual crossover from an energy driven to entropy driven transport of Na + ions with the Na + concentration at the interlayers. The transport mechanism and pathways of the mobile ions also modifies.