How to monitor guest exchange in host-guest systems

The process of guest-exchange in host-guest compounds is reviewed, with a brief discussion of the mechanism and followed by an outline of various analytical techniques, which are commonly employed to follow the progress of the exchange reaction. These include thermal gravimetry (TG), differential scanning calorimetry (DSC), gas-liquid chromatography, nuclear magnetic resonance (NMR), colour change and other more specialised methods. In-situ methods are emphasized and structural changes as monitored by powder X-ray diffraction or single-crystal to single-crystal transformation are considered. An example is given of a guest exchange in an organic system comprising a xanthenol host that initially contains chlorobenzene as guest, which is exchanged with toluene. The procedure is carried out on a single crystal and the structure is monitored as a function of time. The refinement of the structures, which exhibit disordered mixed guests, poses special problems, and a good starting model for the site occupancies of the guest atoms is important. Many such reactions may be carried out using a simple apparatus.