Department of Chemistry (» Postal address)
The main focus is - by definition - on Computational Chemistry and its application of quantummechanical methods. The dramatic increase in computational power in the last few years has led to more and more advanced approaches in ab initio and density functional theory. The accuracy of the methods now allows the application of computational chemistry to a large number of problems in chemistry. In our reasearch, we study asymmetric reactions, structural properties of inorganic complexes, conformational and electrochemical properties of carbohydrates as well as the reactivity of small and potentially unstable molecules. In recent years, this research has been extended to material science and many topics from nano- and biotechnology such as organic nanofibres, bio solar cells or SERS studies of substrates on metal nanoparticles have been successfully addressed.
Cooperations (in alphabetic order)
Prof. Ernst Anders, Institut für Organische Chemie und Makromolekulare Chemie, Universität Jena (deceased in 2016)
Prof. Arne Lützen, Institut für Organische Chemie, Universität Bonn
Prof. Tim Clark, Computer-Chemie-Centrum, Universität Erlangen-Nürnberg
Prof. Henry Strasdeit, Institut für Chemie, Universität Hohenheim
Prof. Werner Uhl, Fachbereich Chemie und Pharmazie, Universität Münster
Prof. Guillermo Diaz Fleming, Universidad Playa Ancha, Valparaiso, Chile
Dr. Greg Qiao, Department of Chemical and Biomolecular Engineering, University of Melbourne, Melbourne, Australien
Prof. Vencatesan Renugopalakrishnan, Harvard Medical School, Harvard, USA
Prof. Horst-Günter Rubahn, South Danish University, Odense, Dänemark
Prof. Curt Wentrup, Department of Chemistry, University of Queensland, Brisbane, Australien
Prof. Katharina Al-Shamery, Prof. Rüdiger Beckhaus, Prof. Jens Christoffers, Prof. Sven Doye, Prof. Jürgen Metzger, Prof. Manfred Weidenbruch (deceased in 2015)
Deutsche Forschungsgemeinschaft, Bonn
Niedersächsisches Ministerium für Wissenschaft und Kultur, Hannover
Universität Oldenburg, Oldenburg
Norddeutscher Verbund für Hoch- und Höchstleistungsrechnen HLRN, Hannover-Berlin
High Performance Computing Centre, University of Queensland, Brisbane
Center for Scientific Computing, Universität Oldenburg, Oldenburg
For the synthetically important stereoselective reactions only the results are known, i.e. the products and their ee values.There is only limited information available on the mechanisms of these reactions. We examine selected reactions with quantunmechanical methods (both ab intio and semiempirical) to gain insight in possible intermediates and transition states. These data allow the explanation of observed selectivities, possible reaction pathways or the prediction of selectivities for modified reactions, based on the calculation of activation energies. One example is the SAMP alkylation, a highly selective C-C bond-forming reaction. Having gained insight into the mechanism, more efficient reactants can be tested theoretically to improve the selectivity without any preparative effort.
Unusual reactions of (hetero-)allenes and -cumulenes are subject to high-level ab initio (G2(MP2SVP)) and DFT investigations. These methods give very reliable predictions of stability, reactivity and properties of molecules involved in reactions such as cycloadditions, ene reactions and sigmatropic rearrangements. The simulation of 13C NMR, IR and other spectra allows identification of an isomer among several possible forms of a compound. The excellent interplay between experiment and theory due to the necessary equipment for handling reactive species in Prof. Wentrup's group allows the prediction of substituents which influence stability and/or reactivity of formed intermediates in the desired way. Furthermore, we are also able to explain surprising experimental results with our theoretical studies.
Employing density functional theory, we try to explain experimentally observed reactivities of organic main group compounds. For example, silylenes and germylenes add to diacetylenes in different ways: reaction of the former leads to bicyclic bissilirenes, while the latter form acetylene-bridged bisgermaethenes.
Another study deals with the isomeric structures of Si4R6 and Ge4R6, respectively.
The work on the main group elements is in collaboration with Prof. Weidenbruch. For further information see Torsten Bruhn's page.
The calculation of titanium fulvene complexes which are synthesised in the group of Prof. Beckhaus is subject of this project. We are in particularly interested in electronic, magnetic and structural properties. For instance, the reactivity of such complexes can be explained in terms of charge distribution in the fulvene ligands. Interactions between central metal atom and the ligands and therefore the bonding situation can be studied with the help of different population analysis approaches.
The calculation of interesting novel aluminium compounds with different modes of coordination of the aluminium atoms gives insight into the bonding properties of these molecules. For example, an Al-Al contact in a heterocubane could be identified as non-bonding despite having a distance significantly below a "normal" aluminium-aluminium bond. Another structure exhibits an electronically remarkable three center two electron bond between two aluminium and a carbon atom. The different complexation properties of carboxylate or triazine groups to gallium compounds are subject to computational studies. We can explain why the former prefer forming bridging complexes while the latter tends to terminal complexation. In general, the ab initio and DFT calculated sructures are in excellent agreement with Xray structures, determined in the group of Prof. Uhl (Münster). The latest example is the excellent agreement of the predicted calculated geometry with the recently determined structure of an alkyl substituted Ga9R9 radical anion.
In a joint project (universities of Oldenburg, Bonn, Odense (Denmark)) a general procedure to synthesize functionalised p-oligophenylenes has been developed. In a diploe-assisted self-assembly process, these molecular building blocks form needle-shaped nanoaggregates with their morphology depending on the growth conditions and on chemical functionalisation of the phenylenes. This enables us to tailor the (non-linear) optical properties of the aggragates. Our quantumchemical calulations have revealed that the molcular properties (such as hyperpolarisabilities and band gaps) correlate well with the collective properties of the nanofibres.
Preceding the synthesis, our computaional design of functionalised oligophenylenes evaluated the desired properties with respect to synthetic accessability. Based on these results, a number of new classes with potential to form fibre-like nanoaggregates is first tested theoretically, then synthesized and finally after a successful growth process the optical properties of the nanoneedles are studied with different laser techniques. This procedure should give access to new hybrid materials from organic nanofibres on inorganic semiconductors with improved and/or new properties which can be used as building blocks in nanophotonic information technologies.
Furthermore, we are interested in the internal structure of these fibres. Currently, there are no information available about the interaction between the monomeric units, also with respect to functionalisation. Methods such as CPMD or DFTB should give us important insight into the growth process of the nanoaggregates.
Photovoltaics will become increasingly important over the next years as the global energy demand is expected to rise by 50%. The development of new cost-effective solar cells is a requirement as silicon-based cells cannot solve the problem due to high costs and limited ressources. Lead by one of the leading experts in bionanotechnology, Prof. Renugopalakrishnan (Harvard Medical School), a joint project has been initiated toward a revolutionary new approach for the use of light-harvesting proteins in "bio solar cells". Research is centered around bacteriorhodopsin (bR) whch is used in genetically modified variations as „Bio Sensitized Solar Cell“ (BSSC). This is in clear contrast to conventional approaches as in „Dye Sensitized Solar Cells“ (DSSC) which make use of toxic ruthenium complexes as light harvesters. Bacteriorhodopsin exhibits high quantum yields and thermal stability which are important advantages together with the low costs for mutant generation and low toxicity compared to DSSC. In addition to the environmentally favourable collector production, the use of nanofibres and -wires and their increased surface to volume ratio should lead to a higher efficiency of the new bio solar cell.
A prerequisite however is a basic understanding how the proteins can be attached to metal (oxide) surfaces without affecting the bR. Theoretical approaches such as model studies on 49Ti-NMR chemical shifts can give insight into the interaction between the proteine and the TiO2(100) surface. But also QM(/MM) MD calculations of larger segments of protein and/or surface will be used to learn about the dynamic behaviour of this system.
The focus in this collaboration with the Polymer Science Group at the Melbourne University is on the development of polymers with unique macromolecular architecture, of hydrogels for biological separations and of biodegradable polymers as scaffolds for soft tissue engineering. Of particular interest are star microgels which can be synthesized in a controlled, two step radical polymerisation using a crosslinker. Further functionalisation but also partial biodegradation can be achieved by utilising functional groups on the linear arm of rhe microgel. In order to control the polymer's architecture, preceding theoretical studies on the influence of different crosslinker are eminent. A distinct property of these microgels is the ability to form films, where the macro structure of the film’s pore sizes can be controlled by the architecture of the microgels. A distinct property of these microgels is their extreme low viscosity which is very important for the coating industry.