Chemistry deals with the composition, structure, and properties of substances and the reactions and transformations that they undergo. Understanding the chemistry of a system requires knowledge of the arrangement of the atoms and their electronic structure. Diffraction and spectroscopic studies using synchrotron radiation are powerful ways to obtain such detail, as illustrated in the range of examples chosen for this chapter. These encompass systems at low temperature or at high temperature and pressure, static and dynamic systems, solids and fluids, fossilised and new materials, the structure and activity of the planet and the structure and activity of an enzyme. 

The first four examples involve diffraction studies, on carbon nanotubes, on the low-temperature solid phases of refrigerant molecules, on sulphur at high temperature and pressure, and on iron reacting with aluminium oxide also at high temperature and pressure. For the nanotubes, a topic of much interest owing to their future use in a number of technological applications, high-energy photons have been used to probe the structure and alignment of the tubes. For the refrigerants, the intermolecular forces that control the thermodynamic properties, and the packing of the molecules in the crystalline state, are of interest. The crystal structures have been solved from high-resolution powder-diffraction data, owing to the difficulties of growing single crystals at the low temperatures of solidification. Investigations at high temperatures and pressures are technically challenging, but become possible with the high brilliance of the beam and expertise on the ESRF high-pressure beamlines. The phase diagram of sulphur has been simplified from performing measurements in situ , indicating that previous assignments have been based on metastable phases, formed on quenching from high pressure and temperature. Chemical reaction between iron and aluminium oxide at high temperature and pressure may influence conditions in the Earth's interior. In situ diffraction studies above 65 GPa and 2000 K confirm that such reactions can occur.

Chemical reactions can also be investigated by X-ray absorption spectroscopy, giving information about the rates of formation and the structure of intermediates. For EXAFS there is no need for the sample to be crystalline and investigation of multi-step reactions in solution is possible. Energy-dispersive EXAFS measurements on the time scale of a millisecond have followed the oxidation of hydroquinone to quinone involving the loss of two protons, and the transfer of two electrons to iron(III) in solution. Even biological systems under near-physiological conditions can now be studied at room temperature, as illustrated by the measurement of rapid-scan EXAFS spectra from the tetra-manganese oxidation complex of the photosynthesis enzyme.

A micro-focussed beam has been used to investigate the oxidation states of sulphur trapped in minute inclusions in olivine crystals from basaltic volcanic magmas. From the XANES spectra, sulphur(IV) is identified, which can be implicated in the continuous release of sulphur dioxide from volcanoes, such as Stromboli or Vesuvius.

The cause of the colour change of fossilised ivory on heating to form the turquoise-blue Mediaeval gemstone odontolite has been a mystery for centuries. The structural and electronic changes responsible have finally been uncovered from EXAFS and XANES experiments.

EXAFS and quantum-chemical methods have been combined to determine the structure of Np(VII) complexes in alkaline solution. By comparing the structures of the Np(VII) and the corresponding Np(VI) complex, it is now possible to explain the reversibility of the Np(VII)/Np(VI) redox couple.