Z. R. Wasserman

Brownian dynamics study of polymer conformational transitions

Conformational transitions of a macromolecule have been studied by computer simulation of the Brownian dynamics of a polymer chain. An activation energy equal to about one barrier height has been found. However, there is a great deal of cooperativity of transitions especially between bonds which are second nearest neighbors.

Molecular orbital calculation of the bond lengths and photoelectron spectrum of disilane

Abstract A 4-31G gaussian basis has been defined for silicon and applied in an ab-initio SCF MO calculation of the bond lengths in disilane. Bond lengths which minimize the total energy for disilane are r2(SiSi) = 2.352 A and re(SiH) = 1.492 A. The corresponding observed bond lengths are 2.331 ± 0.0003 A respectively. The highest four computed orbital energies correspond closely to ionization potentials observed in the photoelectron spectrum.

Molecular SCF calculations to model (111) surface states and relaxation of diamond

Abstract Ab-initio SCF-MO computations in an STO-3G Gaussian basis on a molecular system, designed to model a carbon atom on the (111) surface of diamond, predict that the danglingbond carbon relaxes about 0.10 A toward the bulk from its position on an extension of the diamond lattice. The dangling bond carbon cation relaxes Inward 0.30 A, while the anion relaxes outward 0.10 A. Tentative ways to describe the dangling bond in terms of computed quantities are presented.

The kinetics of conformational transitions: Effect of variation of bond angle bending and bond stretching force constants

Conformational transitions in chain molecules have been shown to proceed via a reaction coordinate which is a localized mode involving rotations about bonds, and also bond angle bending and bond stretching. By investigating the kinetics as a function of the force constants (flexibility) for bond angle bending and bond stretching, the role of the localized mode is probed. The study reported here consists of computer simulations of the Brownian dynamics of chain motions, and of kinetic calculations of rates and reaction modes. The theory accurately predicts the relative effects of force constant variations on transition rates determined by simulation.

Molecular SCF calculations to model (III) surface relaxation of silicon

Abstract Ab-initio SCF-MO computations in a 4–31 Gaussian basis on a molecular system Si 4 H 9 , designed to model a silicon atom on the (III) surface of silicon, predict that the dangling bound silicon relaxes about 0.05 A toward the bulk from its position on an extension of the silicon crystal lattice. This relaxation is much less than earlier empirical estimates.

The Compton profile of urea

The Compton profile of single crystal urea is measured and compared to theoretical calculations of profiles of the molecule, two dimer configurations, and an estimate of the crystal. The major features of the measured anisotropy are predicted by the theoretical calculations for an isolated urea molecule. The effect of the hydrogen bonding between molecules is small.

The spherical Compton profile of neopentane and the carbon–carbon single bond of diamond

The spherically averaged Compton profile of neopentane is measured and also calculated from an ab initio SCF–MO wave function. The measured Compton profile of diamond, corresponding to a carbon–carbon bond (C–C), is compared is empirical C–C profiles computed from experimental and theoretical profiles for neopentane, ethane, and methane. We conclude that the localized bond model works well in all cases examined.

Statistics of the strength of optical fibers

A discussion is presented of means of depicting statistical data on the strength of optical fibers (or other fibers) both graphically and in terms of analytic statistical models. Emphasis is placed on the cumulative hazard which may be interpreted as the density of flaws along a fiber which lead to breaks below a given strength. A portion of the data seems to be fit well by a Weibull distribution. Failures at high strength which do not fit the Weibull plot are to be censored. The applicability of this Weibull distribution at very low strengths is still open to question. Quantitative proof testing, and/or an identification of failure modes, would help clarify the issue.

Correlation Functions for a Mixture of Gaussian Molecules

The Mayer series for the pair correlation functions of the Gaussian mixture has been determined to ninth power of the density (11‐point graphs). The model is a mixture of A and B in which the AA and BB interaction potentials are zero. The AB interaction is such that the Mayer f function, exp (−vAB/kBT) −1, is a negative Gaussian. The direct correlation functions and radial distribution functions are evaluated using Pade approximants. Comparison is made with solutions of the Percus‐Yevick and hypernetted chain equations.

On the structure and stability of bipyramidal B2O3

The structure of bipyramidal B2O3, which has often been postualted to be a major component of B2O3 vapor and liquid, has been computed using a 4–31 Gaussian basis set and the computer program Gaussian‐70. Bipyramidal B2O3, of assumed D3h symmetry, is computed to have an equilibrium B–O bond length of 1.480 A and an angle between boron bonds to oxygen of 62°. Based on computed total energies for boric acid, water and bipyramidal B2O3, the heat of formation of bipyramidal B2O3 in the gas phase from crystalline B2O3 is estimated to be +342.9 kcal/mole, in contrast to the observed heat of vaporization to gaseous B2O3 of +102.5 kcal/mole at 25 °C. We conclude that bipyramidal B2O3 is a very minor component of the B2O3 gas and liquid phases. (AIP)