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We employ molecular dynamics simulations and the reference interaction site model (RISM) integral equation theory to study the solvation structure and solvation thermodynamics of the transfer process from water to a water-urea mixture. Simple positive and negative ions together with uncharged species of the same size are used as crude models for the hydrophilic and hydrophobic groups of a protein. We find that urea preferentially solvates positively charged species. The solvation free energies obtained indicate that larger solutes favor the transfer from water to a water-urea mixture. The decomposition of the transfer free energy into the energetic and entropic terms shows that the energetic part is much larger than the entropic one and tends to dominate the transfer process, supporting the direct mechanism of urea-denaturation. In addition, the effect of urea on the water liquid structure is discussed from the viewpoint of solvation entropy.

The heat capacity of zeolite 4A (also known as LTA, Linde Type A and sodium zeolite A), in the temperature range from 37 to 311 K, is reported. The heat capacity shows no anomalies in this temperature range. Thermodynamic parameters, H, S and G, relative to their values at T=0 K were derived. From these data, we find that zeolite 4A is stabilized by strong enthalpic interactions. Furthermore, its thermodynamic stability results from the strong SiO and AlO bonds in the primary building units, with bond strengths very close to those in other similar materials.

Effects of complexation of dimethyl phosphate anion (DMP-) with countercations Li+, Na+, K+, Mg2+, and Ca2+ on geometry, charge distribution, IR spectra, and conformational changes of DMP- are investigated using DFT calculations. IR spectra alterations caused by conformational change are shown to depend on the ionic strength of a cation and environment.

Adsorption of the phosphate groups on a silica surface is investigated by ab initio calculations of hydrogen-bond complexes of dihydrogen and dimethyl phosphate anions with orthosilicic acid. It is found that the phosphate groups can form strong hydrogen-bonded complexes with the silanols of the silica surface with stabilization energies of ca. −14 kcal/mol per hydrogen bond. The effect of the coordination of the phosphate species by water, orthosilicic acid, and a sodium cation on the geometry and stability of the -sc/-sc conformation is investigated through consideration of 2-fold axial symmetry complexes. Calculations suggest the reduction of the COPO torsion angle of the -sc/-sc conformation of the phosphodiester backbones upon adsorption of molecules containing these moieties on silica compared to the value of this angle found for phosphates in vacuo and in hydrated forms. Both the polarizing and electron density abstracting abilities of silanols cause electron density redistribution, giving rise to ...

The thermal conductivity of model zeolites was investigated using non-equilibrium molecular dynamics calculations. This type of calculation was found to overestimate the thermal conductivity of low-density silica polymorphs. A better reproduction of the experimental results was found for zeolites, and this was related to the lower phonon mean free path. The thermal conductivity of framework silicates was shown to be determined primarily by the vibrations of the continuous oxygen sublattice. Thus, the most drastic suppression of the heat transfer was related to alterations of the O-O distances; for example, a sixfold reduction in thermal conductivity compared to that of siliceous LTA zeolite was found for LTA-A1PO4. Framework cations were shown to affect the heat transfer by changing the vibrational modes of the structural building units of the framework and non-framework counter-cations, by disturbing the oxygen sublattice locally and acting as Rayleigh and resonant scatterers. A model assuming the heat transfer to be due only to non-dispersive acoustic phonons failed to reproduce the dependence of the thermal conductivity on the mass of the cations and the unit-cell dimension, thus suggesting a more sophisticated mechanism of heat transfer to be operative in framework materials. The effect of non-framework non-ionic species on the thermal conductivity was shown to be determined by their effect on the characteristics of the oxygen framework vibrations. Thus, repulsive interactions between the oxygen sublattice and Xe8 clusters, reducing the anisotropy and anharmonicity of the oxygen vibrations, give rise to enhanced heat transfer in LTA-SiO2 at ambient conditions.

Since carbon molecules with geodesic structures have been detected, the question for potential silicon-based analogues of these species has been raised. The existence of these hypothetical `silicon balls', however, has not been ascertained so far in spite of several theoretical as well as experimental attempts to do so. Several theoretical approaches have been put forward in the past to explain the experimentally observed features of Si N clusters and thus to account for the obvious dissimilarity between finite systems based on the isovalent elements silicon and carbon, but no comprehensive growth sequence model has been generally accepted for Si N so far. The present work is aimed at an exploration of closed-shell Si N systems, derived from highly symmetric cage-like geometries, in the size range N ≤26 by means of Hartree–Fock and post-Hartree–Fock procedures. Particular emphasis is placed on the question of the stabilities of these isomers as compared to the corresponding ground states, wherever available. Cohesive energy calculations suggest a tendency towards an increase of cluster stability with the number of constituents for cage-like Si N isomers up to N =10, while a trend towards destabilization with growing size is found for larger clusters.

The MP2(fc)/6-31G∗ structures, energies and frequencies of the trisilaallyl cation (1) and a number of its Si3H5+ isomers reveal considerable differences from the C3H5+ potential energy surface. Unlike the allyl cation, 1 is not a minimum. A “quasi-cyclic” form (7), with a much shorter Si(1)Si(3) separation and aromatic character, is the global Si3H5+ minimum at MP2 and at B3LYP/6-311+G∗∗. However, Raghavachari's (J. Chem. Phys., 95 (1991) 7373) double-bridged isomer (5) is 1.4 kcal mol−1 lower in energy than 7 at the CCSD(T)/6-311+G ∗ //MP2/6-316 ∗ level. Conjugation in the planar C2v 1 and hyperconjugation in the rotated 9 form are decreased relative to the carbon analogs. The remarkable stability of 5 and 7 can be attributed to their effective charge delocalization.

Ab initio calculations have been performed on gallium phosphate clusters: Ga(OH) 4 − , P(OH) 4 + , (HO) 3 GaOP(OH) 3 and Ga 2 P 2 O 4 )(OH) 8 . The influences of the basis set and the level of theory on predicted configurations of the clusters are discussed. Calculated IR spectra have been analyzed and compared with experimental ones. The potential energy surface for the GaO interaction has been used to derive force-field parameters. The latter were applied in the molecular dynamics simulation of the berlinite and low-cristobalite modifications of GaPO 4 .

Molecular and Brownian dynamics simulations are used to investigate the frequency-dependent dielectric relaxation of ferrofluids, with the objective of identifying features characteristic of dipolar chain formation at low densities. It is shown that the presence of chains gives rise to a high frequency band associated with the vibrational motion of dipoles within the chains. This band serves as a "signature" of association and is not present in dipolar fluids at higher, liquid-like densities. A simple theory that traps the basic features of the relaxation behavior is also presented. (C) 2002 American Institute of Physics.

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