A polarizable model for ethylene oxide

A series of interaction models for ethylene oxide are developed for use in molecular simulation of the thermal properties of both the gas and liquid phases. While it is possible to develop nonpolarizable models to accurately generate either the gas or liquid properties separately, it was not possible to do so using a single model for both phases. A polarizable, rigid all-atom model was developed that reproduces the temperature dependence of the second virial coefficient B(T) and the pressure of the liquid at ambient conditions. The model consists of Lennard-Jones and Coulomb interactions between intermolecular atomic sites plus a scalar polarizability located at the midpoint of the line joining the carbon sites. The electrostatic charges and the polarizability are set to match the experimentally determined dipole and quadrupole moments and the molecular polarizability.     

A generalised friction model for the evaluation of angular momentum auto correlation functions

A theory of brownian motion where the friction coefficient is a finite correlation function of molecular torque is used to derive an expression for the angular velocity autocorrelation function, ψ(r). This is related to the orientational a.c.f., φ(t), using an analysis of bandshapes due to Kubo and extended by Shimizu. Both functions φ(t) and ψ(t) contain inherent weaknesses since ψ(t) has a MacLaurin expansion even up to t⁴ only, and π(t) does not reduce to the Kummer function for an ensemble of free rotors in the so-called limit of vanishing friction. Reasons for this behaviour are discussed.     

A finite-nucleus model for relativistic electronic structure calculations using a Douglas-Kroll-transformed Hamiltonian

Summary We report the implementation of a Gaussian finite-nucleus model in the framework of the spin-free no-pair Hamiltonian obtained from the Douglas-Kroll transformation of the no-pair operator with external-field projectors. The finite nucleus regularizes the weak singularity of the wavefunction at the locations of the nuclei and provides a means for efficient exponent optimization using a spinaveraged relativistic one-component operator. We report and discuss basis sets for the gold atom obtained from various optimization procedures, making use of a point nucleus as well as employing various finite-nucleus models.Dedicated to Prof. Dr. Werner Kutzelnigg on occasion of his sixtieth birthday     

A multiscale simulation model for poly(ethylene oxide)

An overview of the recent development of a multiscale simulation of amorphous polymeric materials at the bulk density is presented. Poly(ethylene oxide), (PEO), (CH₃O-[CH₂-CH₂-O]nCH₃) was selected to illustrate the method. The model starts from an ab initio quantum chemistry to obtain the statistical weights of polymer conformation based on the rotational isomeric state (RIS) theory. PEO chains were then mapped to a coarse-grained model using the modified RIS model onto the second nearest neighbor diamond (2nnd) lattice. The average non-bonded interactions were treated by the discretized Lennard-Jones (LJ) potential. Bulk PEO melts with molecular weight up to 8000 g/mol was generated and equilibrated. The on-lattice properties such as molecular size and conformational statistics agree well with the theory. Fully atomistic amorphous PEO models can be obtained by the reverse-mapping procedure to recover the missing atoms. After an energy minimization step, properties including torsional angle distribution, solubility parameter and static neutron scattering structure factor are in good agreement with experimental results.     

A generalised London formula for dispersion coefficients

A generalised London formula is presented for the one-centre calculation of dispersion coefficients of spherically symmetric atoms in S-states in terms of the static multipole polarisabilities and the related orbital excitation energies of the individual atoms. The results obtained for the C₆, C₈, C₁₀ coefficients for H₂ and for the C₆ coefficients for He₂, Li₂, Be₂ and their mixtures are highly encouraging.     

A note on generalised time-lags

Derivations of an expression for the generalised outgoing ‘adsorption’ time-lag pertaining to transport through a slab membrane made by [Rutherford and Do, Review of time lag permeation technique as a method for characterisation of porous media and membranes, Adsorption 3 (1997) 283] and by [Ash et al., Sorption and surface flow in graphitized carbon membranes. II. Time-lag and blind pore character, Proc. Roy. Soc. London, Ser. A, 304 (1968) 407] are not in agreement. Causes for the discrepancy are identified. Two extensions of the generalised procedure are given with particular attention being paid to ΔL, the time-lag difference.     

A vibrational Hamiltonian model for triatomic molecules based on the Kratzer and Poschl Teller potentials

A Hamiltonian model to describe molecular vibrations of triatomic molecules is proposed. The Hamiltonian is based on the use of the Kratzer potential variable for the stretching motions and a perturbed Poschl Teller potential for the bending one. The perturbation and variational treatments to compute the vibrational energies of this Hamiltonian can be developed using a zero-order system that includes part of the couplings between the stretching and bending motions. All the matrix elements involved in these calclations can be then evaluated in closed form. A numerical application to the HCN molecule is made. © 1994 John Wiley & Sons, Inc.     

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

We analyze the low-energy electronic structure of a series of symmetric cationic diarylmethanes, which are bridge-substituted derivatives of Michler's Hydrol Blue. We use a four-electron, three-orbital complete active space self-consistent field and multi-state multi-reference perturbation theory model to calculate a three-state diabatic effective Hamiltonian for each dye in the series. We exploit an isolobal analogy between the active spaces of the self-consistent field solutions for each dye to represent the electronic structure in a set of analogous diabatic states. The diabatic states can be identified with the bonding structures in classical resonance-theoretic models of cyanine dyes. We identify diabatic states with opposing charge and bond-order localization, analogous to the classical resonance structures, and a third state with charge on the bridge. While the left- and right-charged structures are similar for all dyes, the structure of the bridge-charged diabatic state, and the Hamiltonian matrix elements connected to it, change significantly across the series. The change is correlated with an inversion of the sign of the charge carrier on the bridge, which changes from an electron pair to a hole as the series is traversed.     

A chromium catalyst for the polymerization of ethylene as a homogeneous model for the phillips catalyst

A structurally characterized cationic chromium(III) alkyl featuring a bulky nacnac ligand catalyzes the polymerization of ethylene as well as the copolymerization of ethylene with alpha-olefins. This well-characterized homogeneous catalyst constitutes a structural as well as functional model of the widely used heterogeneous Phillips olefin polymerization catalyst.     

Eigenvalues of model Hamiltonian matrices from spectral density distribution moments: The Heisenberg spin Hamiltonian

An approach aimed at approximating the extreme (the lowest and/or the highest) eigenvalues of matrices representing many-electron model Hamiltonians from a knowledge of several spectral density distribution moments is proposed. A detailed discussion of the Heisenberg spin Hamiltonian spectrum is presented as an example of an application. © 1996 John Wiley & Sons, Inc.     

Centrifugal distortion and oxygen-17 quadrupole coupling in ethylene oxide

The microwave spectrum of ethylene oxide has been reinvestigated to determine its quartic centrifugal distortion constants and the quadrupole coupling constants of the ¹⁷O species. The measurements of the quadrupole structure were done in natural isotopic abundance (0.037%) using a computer coupled spectrograph.     

Unit-cell dilation in model ethylene copolymers

Single crystals of model ethylene copolymers have been grown in dilute solution. The effects of narrow molecular weith distribution (MWD) and a perfectly random distribution of ethyl branches (ca. 17 per 1000 main-chain carbon atoms) on the unit cell of this ethylene copolymer analog were examined. X-ray diffraction on dried powder samples revealed expansions of up to 1.5% in a and 0.5% in b. a increased as a mild function of molecular weight; the dependence of b on MW was not as clearly defined. Electron diffraction studies on isolated suspensiongrown single crystals yielded lattice parameters much more strongly expanded. The large expansion in the basal plane of the unit cell is hypothesized to be accompanied by a fore-shortening of the projected c -axis distance. This is accomplished by and orthorhombic-to-hexagonal polymorphic transformation in which the chains are hypothesized to adopt a disordered helical conformation. The driving force for this transformation is electron irradiation damage of the single-crystal specimens. The rapidity of the transformation is believed to be due to the model polyethylene crystals being in a higher-energy, defect-containing condition prior to irradiation.     

Kinetic model of catalytic ethylene oxidation

The kinetics of ethylene oxidation on a pumice supported silver catalyst promoted with chlorine has been studied. A simplified kinetic equation with two parameters is given to describe the process over a wide range of ethylene concentrations. The parameters of the equation have been determined as a function of the temperature.

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A semirigid model for the rotation-bending Hamiltonian in the electron diffraction analysis of triatomic molecules. Cadmium dibromide

Journal of Structural Chemistry -