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.
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.
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.
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.