Crystallography Laboratory University of Nijmegen

Research strategy and program

Position

Crystallography is a member of both the NSR and RIM research schools.

Scope of research

The research done in our group is focused on the application and development of (new) X-ray diffraction methods for the analysis of structures of routine and 'difficult' compounds and the development of methods which translate structural information into new directions for design and synthesis of new compounds and materials.

In the last years our group has participated in many chemical and physical projects which involved identification and determination of structures and structural properties of compounds in the solid state. Although our experimental techniques are focused on properties of the solid state, in most cases the examination of single crystals or crystalline powders by X-ray diffraction also provides an indirect way to obtain information about compounds in solution.
Identification of molecules but also the determination of accurate geometries is very important for chemists and physicist in the process of design, synthesis and examination of new compounds. In most cases our group could give a significant contribution to current research in the form of accurate crystal structures, information from powder diffraction studies and/or studies of structural information from databases.
There is a significant increase is requests for structure determination by single-crystal X-ray diffraction. In 1996 we received 59 requests for crystal structure determination. In 1997 we received 104 requests.

Projects

A new project involves the study of inclusion compounds. Inclusion compounds, like clathrates, only exist in the solid state. X-ray diffraction is a very powerful tool to examine this type of compounds and in combination with molecular modelling it seems possible to optimize guest molecules which form complexes with target (host) molecules more efficiently. An efficient complexation of host molecules is an important technique in the isolation of chemical compounds from mixtures of molecules.

The development of methods which complete a structural model in an automatic way, in the case of 'difficult' compounds (these difficulties may arise from various causes like poor crystal quality, bad experimental data, superstructure effects, low resolution, unknown composition, pseudo-symmetry, etc.), has now more or less come to an end. The work on the so-called 'recycling procedure', which means the automatic completion of partially determined crystal structures, has resulted in a new version of DIRDIF, a computer program system for crystal structure determination by Patterson methods and direct methods applied to difference structure factors. This new version of DIRDIF96, a complete package for structure determination from single-crystal X-ray diffraction data, is also distributed on World Wide Web and has already found its way to many users all over the world.

Strategy

The accurate determination of the interactions that play an important role in the formation of molecular assemblies is an essential step in the process of design and synthesis of new (supra)molecular compounds.
Single-crystal X-ray diffraction is a very powerful method to study molecular interactions in the solid state. Moreover, crystal structures form the basis for molecular modelling studies which improve the understanding of the intermolecular interactions involved. The key-techniques for such structure determinations are single-crystal and powder diffraction in combination with other methods like NMR etc.

In our group the combination of X-ray diffraction methods with molecular modelling has shown to be very succesful in the study of clathrates (inclusion compounds) which are formed by a complexation process. It was possible to obtain a clear insight into the factors which influence a succesful complexation and to find a relation between structural properties of the guest-species and complexation efficiency of the target molecule.
We believe that research on co-crystallizing compounds is a good strategy for obtaining insight into molecular interactions. Such insight may be of significant interest for co-crystallization of supramolecular compounds in order to obtain a crystalline state which would otherwise not be accessible. The approach we have chosen for studying a specific group of complexes can easily be applied to other systems and therefore we wish to continue this theme in the future.

The future

Structure analysis by X-ray diffraction is an essential technique in the study of molecular and nano structures. However, routine methods will probably prove insufficiently powerful in certain fields of supra- and nano structures. The tendency in supra- and nanomolecular chemistry to design systems which (due to various causes like poor crystallinity etc.) can not be analysed in a routine way asks for the application and development of X-ray methods which elucidate structural features of more 'unusual' compounds.

It is well known that medium sized crystal structures (number of atoms > 100) are still hard to solve by single-crystal diffraction techniques. It is expected that, due to the increasing size of the supramolecular systems under study and the poor crystallinity of assemblies held together by non-covalent interactions, there is a need for more powerful techniques and methods for structure determination. In the crystallography group there is a significant expertise in methodology development for single-crystal X-ray diffraction.
The use of modern X-ray equipment (access to CCD diffractometers etc.) and new improved structure solution methods will be essential for studying crystalline supramolecular compounds.

One- and two-dimensional periodicity of compounds can succesfully be studied by powderdiffraction. However, the periodicity of supramolecular systems (large d-spacings) and its temperature dependency (e.g. in liquid crystals) needs state-of-the-art equipment and methods. We believe that application of modern powder-diffraction techniques is essential for studying supramolecular assemblies and that development in this direction is needed.
Insight in one- or two-dimensional periodicity provides useful information about preferred orientation and stacking of molecules which, although no three-dimensional structure is available, may yield essential information about intermolecular interactions.