Stereospecific metallocene catalysts: Scope and limits of rational catalyst design

Structure-performance relationships in the propene polymerization behaviour of a number of silicon bridged bisindenyl zirconocenes show a rational pattern. Prognosis of the polymerization behaviour of different type structures, however, often fails, which is demonstrated by three examples of new zirconocenes. These include two bisindenyl complexes with two-membered silicon bridges and one isospecific bridged fluorenyl cyclopentadienyl species. It is thus demonstrated that the scope of a “rational catalyst design” in the field of metallocene catalysts is still limited.     

Effect of Pressure on the Miscibility of Polyethylene/Poly(ethylene‐alt‐propylene) Blends

Summary: Effect of density, and hence pressure, on the miscibility of a 50:50 mol/mol PE/PEP blend was studied using a coarse‐grained MC simulation approach on a high‐coordination lattice, with the conformations of the coarse‐grained chains constrained by the RIS model. Interchain pair correlation functions are used to assess the miscibility of the mixtures. Miscibility increases with increasing temperature over the range −50–150 °C. It is rather insensitive to pressure at high temperatures, but at −50 °C, the blend miscibility increases with decreasing pressure. The findings are consistent with the fact that the blend is an UCST blend and that the simulation temperatures used, except −50 °C, were considerably higher than the UCST of the blend. The pressure dependence of the blend miscibility observed near −50 °C is also in agreement with the experimental observation that the blend exhibits a negative volume change of mixing. The present work demonstrates that the coarse‐grained MC approach, when it is used with periodic boundary cells of different sizes filled with the same number of chains, is capable of capturing the pressure dependence of UCST blends. In addition, such a simulation also provides us with insights about the molecular origin of the observed pressure dependence of miscibility. In the present case, the segregation of PE and PEP chains at low temperatures and high pressure simply originates from the fact that fully extended segments of PE chains tend to cluster so that their intermolecular interactions can be maximized. As the temperature increases, there is a decrease in the probability of a trans state at a C C bond in PE, and therefore the attraction between the PE chains is reduced at higher temperatures, promoting miscibility and the UCST behavior.

Density (pressure) dependence of the 2^nd shell pair correlation function values for a 50/50 PE/PEP blend at −50 °C.     

Molecular mechanics (MM3) parameterization for oxocarbenium ions

The physical properties of a diverse group of 12 oxocarbenium ions have been studied with ab initio calculations at the MP2/6‐31+G* level of theory. Based on theoretically derived properties such as molecular equilibrium geometry, dipole moment, and vibrational frequencies, a molecular mechanics (MM3) force field has been developed with the assistance of the programs TORSMART and MPMSR, components of our artificial parameter development and refinement method. The MM3 force field is now able to reproduce bond lengths, bond angles, moments of inertia, dipole moments, torsional energy profiles, and vibrational frequencies of oxocarbenium ions, which will allow further studies of glycoside hydrolysis and their rates of reaction. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 329–339, 2000     

A finite element method for free surface flows of incompressible fluids in three dimensions. Part I. Boundary fitted mesh motion

Computational fluid mechanics techniques for examining free surface problems in two‐dimensional form are now well established. Extending these methods to three dimensions requires a reconsideration of some of the difficult issues from two‐dimensional problems as well as developing new formulations to handle added geometric complexity. This paper presents a new finite element formulation for handling three‐dimensional free surface problems with a boundary‐fitted mesh and full Newton iteration, which solves for velocity, pressure, and mesh variables simultaneously. A boundary‐fitted, pseudo‐solid approach is used for moving the mesh, which treats the interior of the mesh as a fictitious elastic solid that deforms in response to boundary motion. To minimize mesh distortion near free boundary under large deformations, the mesh motion equations are rotated into normal and tangential components prior to applying boundary conditions. The Navier–Stokes equations are discretized using a Galerkin–least square/pressure stabilization formulation, which provides good convergence properties with iterative solvers. The result is a method that can track large deformations and rotations of free surface boundaries in three dimensions. The method is applied to two sample problems: solid body rotation of a fluid and extrusion from a nozzle with a rectangular cross‐section. The extrusion example exhibits a variety of free surface shapes that arise from changing processing conditions. Copyright © 2000 John Wiley & Sons, Ltd.     

On the Origin of the Low-Spin Character of Cytochrome P450cam in the Resting State—Investigations of Enzyme Models with Pulse EPR and ENDOR Spectroscopy

The P450 enzyme model 1 is a high-spin system. EPR and ENDOR spectra reveal the coordination of water to the Fe^III center. This is the first experimental proof that coordination of water is not the single determining factor in the stabilization of the low-spin character of the cytochrome P450 resting state.     

Laser ablation inductively coupled plasma atomic emission spectrometry of a uranium-zirconium alloy: ablation properties and analytical behavior

The ablation properties and analytical behavior of a uranium-zirconium alloy have been examined using tandem laser ablation/pneumatic nebulization sample introduction in conjunction with inductively coupled atomic emission spectrometry (LA-ICP-AES). An apparent change in composition of the laser ablation aerosol (1–15 GW cm⁻² Zr deficient, 40–250 GW cm⁻² Zr rich) is observed. This phenomenon is independent of laser wavelength. After collection and bulk chemical analysis of the ablation product, this phenomenon is attributed to an atomization interference in the ICP.

Two distinct modes of laser ablation have been observed which depend upon the wavelength of the ablating laser (visible or near infrared). These two modes result in characteristic ablation crater types and analyte emission behavior. Ablation yields at 1064 nm are dependent upon laser power density only, whilst yields at 532 nm are dependent upon both laser power density and illumination area. The latter is considered to be symptomatic of direct interaction of the laser light with the surface, and the former, of indirect coupling of laser energy, via a micro-plasma, into the surface.