Improvement of methodologies for experimental charge density determination

Abstract

For over 100 years, crystallography has utilised the interaction of X-radiation and electrons within a crystal to enable the reconstruction of a 3-dimensional model of the arrangement of atoms within crystalline solids. Crystallography is considered to be the gold standard of analytical techniques within the chemical and material fields. An atomic model based on the distribution of the electron density is most commonly generated using a number of parameters to describe the atomic arrangement within space. A standard crystallographic experiment assumes that electrons are spherically distributed around atomic nuclei. However, this does not account for electron density within bonding regions, or other classically known features such as lone pairs of electrons. Charge density studies, the pinnacle of structural determinations, involve the addition of parameters to a structural model to characterise the non-spherical density. This can improve the accuracy of the model of the overall distribution of the electron density and allow further investigation into bonding, intermolecular interactions and more advanced structural properties. Some studies into the reproducibility of charge density models have previously been attempted however these are far from exhaustive. The reproducibility, i.e. how well features in the models can be recreated when using different instrumentation and data processing methods, infers the dependability of the parameters derived from models and is therefore paramount. Many different options for processing data exist, including the choice of integration algorithm and if any diffraction data should be excluded from a refinement. These choices may affect the final form of the model of electron density distribution and hence any properties derived from it and therefore need to be approached with caution. Within this thesis an investigation into the effect of processing methods is undertaken using 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. Additionally, newly developed and improved methods to assist the processing and evaluation of charge density are discussed in detail including a program for calculating optimised parameters for a weighting scheme that can be applied to charge density refinements. A number of small molecule single crystal structures have been investigated and their structural models are presented. These highlight the use of the newly developed tools and methodologies along with their effect on high resolution charge density studies.