The Crystal Structure of Ethylenebis(biguanidine)nickel(II) Dichloride Monohydrate

The crystal structure of ethylenebis(biguanidine)nickel(II) dichloride monohydrate, Ni(C_6H_(16)N_(10))Cl_2.H_2O, has been determined and refined on the basis of three-dimensional intensity data collected on an automated diffractometer. The crystals are monoclinic, space group P2_1/c, with cell dimensions ɑ = 6-911, b = 11·678, c = 18·055 Å, β = 101·39°; there are four molecules in the cell. The structure was determined by Patterson methods and refined by least-squares to an R index of 0·048 and a goodness-of-fit of 1·11 for 2879 reflections of non-zero weight. The resulting standard deviations in the atomic positions are about 0·002-0·003 A for the C, N and 0 atoms, 0·02-0·03 Å for the H atoms and less than 0·001 Å for Ni and CI-. The organic ligand is tetradentate and forms a square-planar array about the central nickel atom; the average Ni-N distance is 1 ·865 Å. Chemically equivalent bonds are equal in length within experimental error, and the bond distances have been satisfactorily correlated with molecular-orbital and valencebond descriptions of the cation. All available hydrogen atoms are involved in hydrogen bonds to chloride ions or water molecules. An interesting feature is that, in spite of considerable double-bond character in the C-N bonds, many of the hydrogen atoms are displaced appreciably from the plane of the cation toward the hydrogen-bond acceptors; the bonding about the nitrogen atoms thus becomes pyramidal

Free energy and vibrational entropy difference between ordered and disordered Ni3Al

We have calculated free energy and vibrational entropy differences in Ni3Al between its equilibrium ordered structure and a disordered fcc solid solution. The free energy and entropy differences were calculated using the method of adiabatic switching in a molecular-dynamics formalism. The path chosen for the free-energy calculations directly connects the disordered with the ordered state. The atomic interactions are described by embedded-atom-method potentials. We find that the vibrational entropy difference increases with temperature from 0.14kB/atom at 300 K to 0.22kB/atom at 1200 K. We have calculated the density of states (DOS) of the disordered phase from the Fourier transform of the velocity-velocity autocorrelation function. The disordered DOS looks more like a broadened version of the ordered DOS. Analysis of the partial density of states shows that the Al atoms vibrations are most affected by the compositional disorder. The phonon partial spectral intensities along the 〈100〉 direction show that the vibrational spectrum of the disordered phase contains intensities at optical mode frequencies of the ordered alloy. We find that the volume difference between the ordered and disordered phases plays the most crucial role in the magnitude of the vibrational entropy difference. If the lattice constant of the two phases is set to the same value, the vibrational entropy difference decreases to zero

The Crystal Structure of Ethylenebis(biguanidine)nickel(II) Dichloride Monohydrate

The crystal structure of ethylenebis(biguanidine)nickel(II) dichloride monohydrate, Ni(C_6H_(16)N_(10))Cl_2.H_2O, has been determined and refined on the basis of three-dimensional intensity data collected on an automated diffractometer. The crystals are monoclinic, space group P2_1/c, with cell dimensions ɑ = 6-911, b = 11·678, c = 18·055 Å, β = 101·39°; there are four molecules in the cell. The structure was determined by Patterson methods and refined by least-squares to an R index of 0·048 and a goodness-of-fit of 1·11 for 2879 reflections of non-zero weight. The resulting standard deviations in the atomic positions are about 0·002-0·003 A for the C, N and 0 atoms, 0·02-0·03 Å for the H atoms and less than 0·001 Å for Ni and CI-. The organic ligand is tetradentate and forms a square-planar array about the central nickel atom; the average Ni-N distance is 1 ·865 Å. Chemically equivalent bonds are equal in length within experimental error, and the bond distances have been satisfactorily correlated with molecular-orbital and valencebond descriptions of the cation. All available hydrogen atoms are involved in hydrogen bonds to chloride ions or water molecules. An interesting feature is that, in spite of considerable double-bond character in the C-N bonds, many of the hydrogen atoms are displaced appreciably from the plane of the cation toward the hydrogen-bond acceptors; the bonding about the nitrogen atoms thus becomes pyramidal

Exploring the boundary between atoms and the continuum by computers: a personal history

In this admittedly personal account of the history of atomistic simulations of fluids (at the atomic or molecular level), I will focus on the competing efforts to reach the boundary between atoms and the continuum. The prevailing wisdom was that thermal fluctuations at the atomistic scale—both time (a few mean collision times) and space (a few atomic spacings)—would make the connection virtually impossible. This is just a part of the story about how molecular dynamics was able to connect to Navier–Stokes–Fourier hydrodynamics. Resistance in the theoretical physics community to computer simulations of equilibrium fluids at the atomistic scale was only exceeded by the even stiffer objections to non-equilibrium molecular-dynamics simulations: after the fifty years from Boltzmann to molecular dynamics, it took another quarter century to overcome the doubts

LA-6146-MS Informal Report NRC-8 The Interaction between Cesium and Graphite for Use in the Study of Surface Phenomena THE INTERACTION BETWEEN CESIUM AND GRAPHITE FOR USE IN THE STUDY OF SURFACE PHENOMENA

ABSTRACT Surface diffusion has been hypothesized as the fast mode of an unusual fast-slow, two-mode transport process that has been observed in recent diffusion experiments with cesium in graphite. An interaction potential between a cesium aton and a graphite surface is obtained in order ts study this surface diffusion by computer simulation (molecular dynamics method). At low surface coverage, the Interaction between cesium atoms can be ignored so that the motion of only one cesium atom need be followed, albeit in a very complicated potential energy surface. Cesium is spontaneously ionized by graphite, so that the interaction of cesium with the graphite surface contains pairwise Cs* -C terms (valence, induction, and dispersion forces) as well as an image-charge model of the bulk electrostatic interaction. All parameters but the strength of the repulsive Cs* -C force are obtained by theoretical estimates, while this last parameter is determined by requiring that the adsorption Cs* -C bond length be the -same as observed in cesium-graphite lamellar compounds. Results indicate that the adsorption energy for a pit in the graphite surface of one to five missing carbon atoms is not greatly increased over that for the perfect surface (the one-atom hole is slightly repulsive compared to the perfect surfa'"" 1 For the hexagonal six-atom pit, the adsorption energy increases dramatically from about 120 kcal/mole for the perfect surface to about 200 kcal/mole and remains essentially constant for larger holes. Results for potential energy barriers to migration indicate that a cesium ion on a perfect graphite surface moves along the surface like a free particle at temperatures above 1000 K. Thus, truly diffusive or randomwalk behavior at such temperatures requires the presence of defects in the graphite surface

Heat-flow equation motivated by the ideal-gas shock wave

We present an equation for the heat-flux vector that goes beyond Fourier's Law of heat conduction, in order to model shockwave propagation in gases. Our approach is motivated by the observation of a disequilibrium among the three components of temperature, namely, the difference between the temperature component in the direction of a planar shock wave, versus those in the transverse directions. This difference is most prominent near the shock front. We test our heat-flow equation for the case of strong shock waves in the ideal gas, which has been studied in the past and compared to Navier-Stokes solutions. The new heat-flow treatment improves the agreement with nonequilibrium molecular-dynamics simulations of hard spheres under strong shockwave conditions. © 2010 The American Physical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Microscopic Simulations of Complex Hydrodynamic Phenomena: Proceedings of a NATO ASI

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