Density and temperature dependence of proton diffusion in water: a first-principles molecular dynamics study.
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Computer Simulation↗
Biomedical subjects
Tamio Ikeshoji
Publications and source records attributed to Tamio Ikeshoji.
Hydrogen bond driven chemical reactions: Beckmann rearrangement of cyclohexanone oxime into epsilon-caprolactam in supercritical water.
Recent experiments have shown that supercritical water (SCW) has the ability to accelerate and make selective synthetic organic reactions, thus replacing the common but environmentally harmful acid and basic catalysts. In an attempt to understand the intimate mechanism behind this observation, we analyze, via first-principles molecular dynamics, the Beckmann rearrangement of cyclohexanone oxime into epsilon-caprolactam in supercritical water, for which accurate experimental evidence has been reported. Differences in the wetting of the hydrophilic parts of the solute, enhanced by SCW, and the disrupted hydrogen bond network are shown to be crucial in triggering the reaction and in making it selective. Furthermore, the enhanced concentrations of H(+) in SCW play an important role in starting the reaction.
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Size- and temperature-dependent structural transitions in gold nanoparticles.
Size- and temperature-dependent structural transitions in gold nanoparticles were revealed with morphology statistics obtained by high-resolution electron microscopic observations for thousands of particles annealed in a helium heat bath. We found that gold nanoparticles over a wide size range, 3-14 nm, undergo a structural transformation from icosahedral to decahedral morphology just below the melting points. It was also clarified that the formation of bulk crystalline structures from the decahedral morphology requires the melt-freeze process due to an insurmountable high free-energy barrier.
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First-principles molecular-dynamics simulations of a hydrated electron in normal and supercritical water.
A first principles study of a hydrated electron in water at ordinary and supercritical conditions is presented. In the first case, the electron cleaves a cavity in the hydrogen bond network in which six H2O molecules form the solvation shell. The electron distribution assumes an ellipsoidal shape, and the agreement of the computed and the experimental optical absorption seems to support this picture. At supercritical conditions, instead, the H-bond network is not continuous and allows us to predict that the electron localizes in preexisting cavities in a more isotropic way. Four water molecules form the solvation shell but the localization time shortens significantly.
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Microscopic pressure tensor for hard-sphere fluids.
The microscopic pressure tensor, which is not uniquely defined, is analyzed for a uniform hard-sphere fluid in Cartesian and spherical coordinates. Two popular definitions, one due to Irving and Kirkwood (IK) [J. H. Irving and J. G. Kirkwood, J. Chem. Phys. 18, 817 (1950)] and the other due to Harasima (H) [A. Harasima, Adv. Chem. Phys. 1, 203 (1958)], were used in this work. The IK definition is found to give the same ensemble average of the local pressure in Cartesian and spherical coordinates for a homogeneous hard-sphere system. The pressure obtained from the H definition gives, on the other hand, different results in the two coordinate systems. In Cartesian coordinates, the H pressure is identical to the IK pressure, but in spherical coordinates, the pressure depends on R (the distance from the origin). Therefore the H definition does not give a proper pressure tensor.
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