Norah V. Cohan

Molecular Orbital Study of Ionic Defects in Ice

The potential energy for the diffusion of positive and negative defects in ice is calculated by a SCF—MO—LCAO procedure. The resulting height of the potential energy barrier for positive defects is much lower than for negative ones, thus explaining qualitatively the greater mobility of the H3O+ ions. It is also found that the potential‐energy heights are very sensitive to the distance between the oxygens, the diffusion being greater when the oxygens are nearer. The results obtained suggest therefore that the diffusion of positive defects is correlated to the vibrations of the lattice.

Cluster calculation on the interaction and discharge of ions on a silver surface

Abstract The interaction of halogen and alkali ions with the (100) surface of silver was studied at very short distances. Calculations were performed by the iterative extended Huckel method (IEHT) for (Ag n X) ± clusters with n =9 and n =37 and X=F, Cl for the negative and X=Na, K for the positive clusters. A careful analysis of the parameters used in the IEHT was performed and the main results obtained are: Cl − ions lose more of their negative charge than F − ions on approaching the (100) silver surface, and the electronic charge thus transferred to the silver cluster is distributed as far as possible from the adsorbed species. Both halogens form localized bonds with the nearest Ag atoms, which are very similar to those for the neutral adsorbed species. The interaction of the alkali ions with silver is very small and it decreases as the size of the cluster increases, supporting the idea that the alkali ions interact only via a solvation shell.

Molecular dynamics study of two dimensional and adsorbed microclusters

Two dimensional and adsorbed microclusters of six, seven, eight, and nineteen rare gas atoms are studied by the molecular dynamics method. The behavior of all these clusters as a function of temperature presents two distinct regions: a low temperature regime typical of the vibrations in a solid and a high temperature or liquid one in which the clusters remain condensed but the particles move in very different ways, depending on the number of atoms and the shape of the cluster. Systems having close shells of neighbors of a central atom (those of seven and nineteen atoms) also show a transition region and at higher temperatures other properties typical of the liquid state. The effect of a weak interaction with a surface becomes important at high temperatures where the clusters evolve to three dimensions as some of the atoms move away from the surface.

On the hydrogen bond in an ice-like structure

Abstract The interactions of a water molecule with its first and second neighbors in an ice-like structure are studied, and several definitions and methods of calculation are compared. An attempt is made to calculate the cooperative effect which is found to be smaller than the thermal energy kT at 0°C.

Structure of the hydrated electron

We study the short range interactions between an electron and the neighbor water molecules, with a modified version of the CNDO/2 method. The basis set used includes excited atomic orbitals which are fundamental in the study of the system (H2O)n−. Our calculations indicate that the electron is not associated to a vacancy or other type of defect but to a group of water molecules in the normal microscopic structure of ice and liquid water. In ice it can also be considered as an extra electron in an otherwise perfect crystal lattice (an electronic polaron). Our results are consistent with the hydration energy, spectra, and photobleaching experiments.

A quantum electronic polaron model of the hydrated electron

Abstract The ground state energy of the hydrated electron in low temperature ice, ΔG ice , is calculated on the assumption of a quantum electronic polaron model. The Hartree-Fock energy of the bottom of the conduction band, ϵ HF , is estimated from a previous work and the correlation energy, ϵ corr , is calculated by second order perturbation theory using Toyozawas interaction hamiltonian. The theoretical value so obtained for ΔG ice = ϵ HF + ϵ corr is compared with the experimental results on photoemission into electrolyte solutions.

VALENCE BOND TREATMENT ON THE B STATE OF THE HYDROGEN MOLECULE

The potential energy of the lowest 1 Σu+ state of the hydrogen molecule is given as a function of the internuclear distance. Calculations were carried out by the valence bond method using Slater 1s and 2p atomic orbitals. Both ionic and covalent structures were considered. The 1s orbitals of the ionic and covalent functions included a variational parameter but the orbital exponent of the 2p orbital was kept constant. It is found that the introduction of the 2p orbital into the wave function results in a considerable improvement of the dissociation energy and that further the state is essentially covalent in character rather than ionic, as previously reported.

A comparison between electrons solvated in ammonia and water: The volume expansion

Abstract We suggest that the large difference in volume expansion for the solvated electron in ammonia and water is due to the different intermolecular interactions in both liquids. Calculations with the CNDO/2 method for groups of ammonia molecules assuming a polarization due to independent molecular rotation give a reasonable value for the volume expansion of the ammoniated electron. In water, strong hydrogen bonds prevent independent molecular rotations and therefore no volume expansion is obtained.

Charge distribution in structurally disordered systems

Abstract In this paper we calculate the charge distribution n(Q) for a structurally disordered system of identical atoms. The atoms have non-zero charges associated to them only because the spatial configuration around each atom is different. The systems considered are those for which an atomic basis set is adequate and an iterative tight binding scheme, where the matrix elements depend on the atomic charges, is used. We study the effect of including explicitly the electrostatic interaction among the charges associated to the atoms in the calculation of n(Q). We propose that the atomic positions of a totally random configuration be modified by amounts proportional to the electrostatic forces on the atoms. We call this a relaxation effect. We find that the new atomic configurations give a narrower n(Q) although they have practically the same energy and radial distribution function as the original configuration.

Calculation of the density of states of NbNx by the recursion method

Abstract The density of states, n ( E ), of NbN x has been calculated by means of the Recursion Method for x ⪡ 1. The results have been compared with experiment, obtaining that the most important trends of the density of states are reproduced fairly well, even if the variation of the density of states at the Fermi level, as a function of the concentration of N -impurities cannot be predicted.