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    Chemistry

Text "Chemistry"	Sigrid Peyerimhoff, Theoretical Chemistry, University of Bonn
Structure and Reactivity of Sulphated Zirconia Catalysts	Frank Haase and Joachim Sauer, Quantum Chemistry Working Group, Humboldt University, Berlin
Structure and Dynamics of Substitutionally Doped Fullerenes	Isabelle Billas, T. Patrick Martin, Mauro Boero, Michele Parrinello, Max Planck Institute for Solid State Research, Stuttgart; Carlo Massobrio, CNRS, Strasbourg
Molecular Dynamics of Pyrazine after Excitation with Ultraviolet Light	Andreas Raab, Graham Worth, Hans-Dieter Meyer and Lorenz Cederbaum, Theoretical Chemistry, University of Heidelberg
Single-Molecule Atomic Force Microscopy Simulations on Antibody/Antigen Complexes	Berthold Heymann and Helmut Grubmüller, Max Planck Institute for Biophysical Chemistry, Göttingen
Monte Carlo Simulation of Liquid Trifluoromethane	Matthias Hloucha and Ulrich Deiters, Institute of Physical Chemistry, University of Cologne

Text "Chemistry"

Chemistry, that is the investigation of the properties of substances and their transformations into each other, has been based for centuries on a meticulous observation of nature and on imaginative experiments. Knowledge of the structure of matter composed of positively and negatively charged nuclei and electrons and the development of quantum mechanics permitted the formulation of a mathematical equation from which, in principle, all the observable properties of atomic and molecular systems can be determined. This paved the way for a mathematical treatment of chemical processes. However, this equation is so complicated that only a solution by approximation is possible for a many-particle system. Promoted by the rapid development of computers, this field of quantum chemistry has undergone a rapid development since the sixties. Calculations today permit reliable statements on chemical systems with a precision entirely comparable to that of experimental data. Such calculations can also provide information about systems or processes not directly accessible to experiments. Nevertheless, experiments are not superfluous; the methods and theoretical predictions are improved in a constant dialogue between experiment and theory.

The methods used range from quantum chemical "ab initio" techniques, aiming at a solution of quantum chemical equations without empirical data, through density functional methods using a few parameters, up to more approximative methods of molecular mechanics or molecular dynamics using classical equations of motion and force fields obtained empirically or by quantum mechanics. Depending on system size and computer effort, various "hybrid techniques" are also applied.

An important field of application is the investigation of chemical reactions on the molecular level. Microscopic understanding facilitates selective intervention in a reaction process - and this is ultimately the aim of many experimental and theoretical efforts in the field of catalysis. The investigations of the dissociation of sulphuric acid on zirconia or the behaviour of fullerenes when incorporating guest atoms into the carbon cage may serve as an example.

Spectroscopy and photochemistry involving the interaction of matter with light from the infrared up to the X-ray region are no longer conceivable without quantum chemical calculations. They can be applied at least for small molecules in order to predict the energy region in which the molecule absorbs, how long the isolated molecule lives and what reactions are possible. This problem area also includes the molecular dynamics of pyrazine.

Simpler methods are used for systems with many thousands of atoms. Their application is currently found primarily in biochemistry, as shown by the example of the antibody/antigen complex, or in modelling liquids.

The award of the 1998 chemistry Nobel prize to Walter Kohn and John A. Pople acknowledges the path-breaking achievements of these scientists in the field of quantum chemistry, but also documents the significance of this domain of research for chemistry as a whole. Hardly any field from molecular physics through inorganic and organic chemistry up to and including biochemistry can today dispense with the application of computer methods. The provision of powerful computers is an essential prerequisite.

(Sigrid Peyerimhoff, Theoretische Chemie, Universität Bonn)

Structure and Reactivity of Sulphated Zirconia Catalysts

The surface treatment of zirconia by sulphate yields an extremely active catalyst for many industrially important chemical reactions. The hitherto unknown configuration of the surface of these sulphur species has now been clarified for the first time by periodic quantum chemical calculations. It was shown that sulphuric acid dissociates completely on zirconia surfaces forming doubly (top) as well as triply (bottom) coordinated sulphate ions and surface hydroxyl groups. The vibration frequencies calculated are in very good agreement with observed spectra and support the experimental assumption that both structures exhibit characteristic infrared absorptions which permit a differentiation. The reactivity of this catalyst is the subject of current investigations.

(Frank Haase and Joachim Sauer, Quantum Chemistry Working Group, Humboldt University, Berlin)

Structure and Dynamics of Substitutionally Doped Fullerenes

The cage-like carbon molecules known as fullerenes have a large number of interesting properties which can be modified by the substitution of skeleton atoms or the incorporation of guest atoms in the cage void.

The substitution of homologous silicon (red in the picture) for a carbon atom in the C₆₀ "football molecule" leads to a distinct local deformation of the originally almost perfect spherical shape and to a change of the electronic energy levels by symmetry reduction. The strong localization on silicon of the highest occupied and the lowest unoccupied molecular orbitals makes them interesting reactive centres. Energy calculation and structure optimization were performed on a CRAY T3E parallel computer by using the Car-Parrinello method which directly couples the atom motion treated by classical mechanics with a quantum mechanical density functional calculation of the electronic energy.

(Isabelle Billas, T. Patrick Martin, Mauro Boero, Michele Parrinello, Max Planck Institute for Solid State Research, Stuttgart; Carlo Massobrio, CNRS, Strasbourg)

Molecular Dynamics of Pyrazine after Excitation with Ultraviolet Light

Pyrazine is an organic molecule resembling benzene. If this molecule is irradiated with ultraviolet light, part of it is absorbed. The absorption process is characteristic of the irradiated molecule and is described by its absorption spectrum.

Quantum mechanical methods are necessary to understand this absorption process. Irradiation sets pyrazine in motion and it performs complicated vibrational motions. The complexity of these dynamics is caused essentially by the intersection of the highly dimensional potential surfaces of two different electronic states. A cross-section of these potential surfaces is shown in the picture.

The dynamics of the pyrazine molecule on the two coupled potentials was calculated on a CRAY T90 vector computer using a new quantum dynamic method developed by the working group.

(Andreas Raab, Graham Worth, Hans-Dieter Meyer and Lorenz Cederbaum, Theoretical Chemistry, University of Heidelberg)

Single-Molecule Atomic Force Microscopy Simulations on Antibody/Antigen Complexes

Molecular dynamics simulations permit the study of the interaction between biological molecules on the basis of atomistic models. Recently developed single-molecule atomic force microscopy techniques allow experimental verification of the simulations.

The picture shows a simulation system for studying an antibody/antigen binding/unbinding process. To this end, in the computer model the antibody (red) with bound antigen (green) was dissolved in a water droplet (blue) containing sodium and chlorine ions (yellow) at physiological concentration. This system comprises a total of 44,571 atoms.

To simulate the atomic force microscopy experiments, the green antigen molecule was subjected to additional forces together with the usual molecular interactions; these additional forces simulate the cantilever of an atomic force microscope which pulls the antigen out of the bonding pocket. The simulations permit an observation of reaction pathways of the antigen molecule out of the binding pocket and of the simultaneous dynamics of the binding pocket and the associated network of hydrogen bonds.

(Berthold Heymann and Helmut Grubmüller, Max Planck Institute for Biophysical Chemistry, Göttingen)

Monte Carlo Simulation of Liquid Trifluoromethane

Trifluoromethane is gaseous at room temperature and regarded as a possible substitute for chlorofluorocarbons. The description of the liquid phase from the melting point to the critical point using an atomistic model is at the centre of these investigations. For this purpose, a pressure and temperature independent model with polarizable particles is used.

The Monte Carlo method allows the calculation of thermodynamic properties by statistical averaging over particle configurations distributed according to given boundary conditions (here: fixed number of particles, fixed pressure and temperature). The intermolecular interaction model adequately reproduces the liquid structure (picture), its density and vaporization heat and the dielectric constant in the entire temperature range.

(Matthias Hloucha and Ulrich Deiters, Institute of Physical Chemistry, University of Cologne)

Introduction	Super- computing	Astro- physics	Elementary Particles	Many Particles	Polymers	Chemistry	Environment	Other Fields