This chapter provides a background on the technique of structure determination through X-ray crystallography and discusses its importance in the context of modern chemistry. It explains the basis of X-ray crystallography through an optical analogy that outlines the main relevant properties of crystalline materials and aspects of symmetry. It also examines the properties of the diffraction pattern of a single-crystal in relation to features of interest in the crystal structure. The chapter covers the geometry of diffraction, symmetry observed in the pattern, and the variation of intensity in the discrete diffraction measurements. It cites available sources of X-rays for crystallography.
Fundamentals of X-ray crystallography
This chapter covers the core subject of X-ray single-crystal diffraction for the determination of chemical compounds. It analyses the use of neutrons instead of X-rays and of powder diffraction. It also discusses the applications to biological macromolecules and crystal structure prediction. The chapter explains how X-rays are used for crystal structure determination, in that they have wavelengths comparable to the separations between atoms in molecules and give measurable diffraction effects from crystals. It refers to neutrons generated by a nuclear reactor or a neutron spallation source in which the associated wavelengths lie in the same range as X-rays, which causes a beam of neutrons to be diffracted as a particulate radiation by crystalline material.
X-Ray Crystallography gives an account of the technique, emphasising its wide-ranging practical application to engineering and the physical and biological sciences. The first chapter looks at the fundamentals of X-ray crystallography. Chapter 2 looks at X-ray crystallography in practice. The third chapter contains a number of case studies, while the final chapter looks at other related topics.
X-ray crystallography case studies
This chapter illustrates the process of X-ray crystal structure determination through a series of examples drawn from a wide range of structural chemistry research. It analyzes the complex [Et4N] [Hg(SR)3], which is an empirical formula corresponding to a monomeric structure. It also addresses the key questions that are answered by crystal structure determination regarding the anion that can be a dimer with bridging thiolate ligands, a higher oligomer, or a polymer. The chapter talks about the equivalence of different isotopes of an element in X-ray diffraction and the determination of the absolute structure of a chiral molecule by resonant scattering effects. It outlines the occurrence of disorder that can be modelled by the use of atoms with partial occupancy in alternative positions.
X-ray crystallography in practice
This chapter highlights the various steps of a typical crystal structure determination, and describes a number of commonpotential problems together with ways of dealing with them. It shows an outline of crystal structure determination in a simplified form known as a schematic flowchart. It also analyzes the intensities of X-rays diffracted by a crystal that depend on the crystal size, unit cell volume, and types and numbers of atoms in the unit cell. The chapter talks about diffracted intensities that are directly proportional to the crystal volume and the absorption of X-rays by crystals, which increases exponentially with crystal dimensions. It explains how the amount of absorption depends on the X-ray wavelength and on the chemical composition.