Stefan Brode

Intermolecular potentials for ammonia based on the test particle model and the coupled pair functional method

The intermolecular interaction ΔE in (NH3)2 is investigated on the SCF level, with inclusion of correlation effects by means of the CPF method and within the simple test particle model. Whereas the linear hydrogen bonded structure is favoured on the SCF level, ΔE = -7·65 kJ mol-1, the most stable geometry on the highest level of theory is a cyclic structure, ΔE = -12·96 kJ mol-1. The minimum is very shallow and allows for appreciable angular motions. The test particle model reproduces the general features of ΔE but shows deviations in details. The computed potentials are used in MD simulations to compute static and dynamic properties of liquid NH3. Good agreement with available experimental results is obtained throughout.

Parallel direct SCF and gradient program for workstation clusters

A parallel direct SCF and gradient program for workstation clusters has been implemented on the basis of the ab initio program package TURBOMOLE. Applications on large molecular systems monitor an appreciable speedup in residence time.

Diastereoselective Synthesis of Chiral (Triazolinylidene)rhodium Complexes Containing an Axis of Chirality

Deprotonation of chiral triazolium salts 1 and reaction of the resulting nucleophilic carbenes with [(COD)RhCl]2 or [(NBD)RhCl]2 afforded square-planar complexes 2–6 in yields of 65–95%. The complexes contain an axis of chirality and a diastereomeric excess of up to 97% was achieved. The relative and absolute configurations of these complexes were determined by NMR spectroscopic investigations and X-ray structure analysis. The application of the rhodium(COD) complexes as catalysts in an asymmetric hydrosilylation reaction has been examined, resulting in enantiomeric excesses of up to 44%. Similar results were achieved for aromatic and aliphatic ketones and a nonlinear temperature effect (principle of isoinversion) was observed.

An optimized MD program for the vector computer cyber 205

Abstract The potential of supercomputers for MD simulation runs is explored for the CYBER 205 vectorcomputer in the treatment of molecular liquids with site-site interaction potentials. Careful vectorization and optimization of remaining scalar codes is shown to reduce the CPU time to 7% as compared to the original code. The advantage of avoiding IO completely by computing time correlation functions in the course of the MD run is demostrated. Special emphasis is given to optimized data structures and data flow in order to minimize start up times and to exploit the hardware features of the CYBER 205.

Zinc Silicates: Very Efficient Heterogeneous Catalysts for the Addition of Primary Alcohols to Alkynes and Allenes

There is an astonishing parallel between the mechanism generally accepted for the addition of water to CO2 catalyzed by the enzyme carbonic anhydrase and the mechanism calculated for the addition of methanol to allene catalyzed by the naturally occurring zinc silicate hemimorphite. The latter reaction was investigated in detail following the observation that hemimorphite as well as an amorphous zinc silicate prepared in situ are excellent heterogeneous catalysts for the addition of primary alcohols to alkynes and allenes [Eq. (1)].

A new rigid motion algorithm for MD simulations

Abstract A new algorithm is developed to integrate Newtons equations for rigid molecules in MD simulations. The procedure integrates the equations for the center of mass and the principal axes of the moment of inertia tensor with the aid of the Verlet algorithm. Explicit equations are derived and complications of the method of constraints are avoided. The new method is fast and stable as is demonstrated in applications for NH 3 , H 2 O and CF 4 .

Excess thermodynamic properties of liquid mixtures of methane and perfluoromethane

New intermolecular potentials, derived in part from ab initio calculations, have been used in molecular-dynamics simulations of liquid mixtures of CH4 and CF4. Results for the excess volumes and excess enthalpies are found to be in excellent agreement with experimental data. Structural features of the simulated mixtures are strongly suggestive of a tendency to demix.

Zinksilicate: hochwirksame heterogene Katalysatoren für die Addition primärer Alkohole an Alkine und Allene

Eine erstaunliche Parallele existiert zwischen dem allgemein anerkannten Mechanismus fur die vom Enzym Carboanhydrase katalysierte Addition von Wasser an CO2 und dem Mechanismus, der fur die Addition von Methanol an Allen, katalysiert durch das naturlich auftretende Zinksilicat Hemimorphit, berechnet wurde. Die letztgenannte Reaktion wurde genau untersucht, da sowohl Hemimorphit als auch ein in situ hergestelltes, amorphes Zinksilicat ausgezeichnete heterogene Katalysatoren fur die Addition primarer Alkohole an Alkine und Allene sind [Gl. (1)].

Ab initio SCF calculations on low-energy conformers of cyclohexaglycine

Results from ab initio self‐consistent field (SCF) calculations with a 3‐21G and a double‐zeta‐plus polarization (DZP) basis set on four low‐energy conformations of cyclohexaglycine are reported. In agreement with results from semiempirical and molecular mechanics force field calculations, the lowest‐energy conformation found at the DZP level is a conformation forming six C7 turns. However, the energy difference to the β‐turn conformers is significantly smaller at the ab initio DZP level than calculated by the other methods. In contrast to the results obtained with some of the other methods, the present ab initio calculations show that both the double‐type‐I β turn and the double‐type‐II β‐turn conformer of cyclohexaglycine are stable low‐energy structures.