Titel:
Development and Applicability of Advanced Methods for Solving 
'Difficult' Crystal Structures by X-ray Analysis

Auteur:
drs. Olaf Randolph Israël

Jaar:
2000

Trefwoorden:
X-ray analysis, diffraction, reflection, direct methods, triple product,
scale factor, crystal structure, triplet, doublet, R2 factor, reliability
index

Abstract:
Single-crystal X-ray diffraction is a method that provides accurate 
structural information on the arrangement of atoms and molecules in 
crystals.
The method is based upon crystals diffracting X-rays only in specific 
directions. The diffracted beams are called reflections, of which the 
directions and the intensities can be measured. The directions contain
information regarding the geometry of the unit cell. Comparison of the
intensities of the separate reflections gives information about the 
diffraction and crystal symmetry. If also the phases of the individual
reflections were known, the exact positions of the atoms in the unit 
cell could be calculated. The lack of phase information is called the 
phase problem and gave rise to the development of many mathematical and
computational methods for its solution.
A powerful aid for solving the phase problem is the computer program 
DIRDIF, which has been developed by the Crystallography group of the 
University of Nijmegen under supervision of Prof.Dr. P.T. Beurskens. 
In this program system several of these methods are successfully 
combined so that most of the small and medium sized structures can 
easily be solved. 
Nevertheless there will always be structures which are not easily 
'cracked'. Some of these structures may suffer from e.g. 
superstructure or aperiodic effects. Others, e.g. supramolecular 
structures, may suffer from poor crystal quality and poor experimental
data. This thesis describes a study for solving difficult crystal 
structures suffering from the above mentioned effects