Selectivity in host-guest compounds

It isn't easy to separate structurally similar guests by conventional methods such as distillation if the mixture components have similar functional groups and boiling points. This thesis aimed to investigate the relationship between the internal structures of host-guest compounds and the selectivity shown by the host molecule when presented with a guest mixture. Four host compounds were used to prepare 29 new inclusion compounds. Two of these hosts were similar bulky diols and were synthesised by Weber (H1 and H2). The other two, a Nibased Werner-type host (H3) and a brominated host (H4), were synthesised by known methods. In addition, two new structures of the host, H1, alone are reported. Inclusion compounds were prepared by recrystallising the host from liquid guests or exposing the powdered host to guest vapours. The internal structure of the resulting compounds was determined by single crystal X-ray diffraction (XRD), and the secondary interactions between host and guest were carefully studied. The thermal stability of compounds was measured using differential scanning calorimetry, and thermal gravimetry – the latter of which also confirmed the ratio of host to guest within compounds. The preference of a host for one guest over another was determined by exposing the host to mixtures of guests, waiting for crystals to form, and then subjecting these crystals to 1H nuclear magnetic resonance (NMR) spectroscopy or gas chromatography (if guest molecules were indistinguishable by NMR). In some cases, the kinetics of host-guest compound formation and guest exchange was monitored by powder XRD, using an in-house custom-built sample holder. These techniques allowed for the measurement of the relative proportions of guests in the crystals, which was then compared to the ratio of guests from which the host was initially recrystallised. In some cases, it was found that the thermal stability of host-guest compounds correlates with the guest preference of the host. I.e., the guest that formed the most thermally stable ...