INCLUSION COMPOUNDS : STRUCTURE AND KINETICS OF DECOMPOSITION
Mino Caira, Anita Coetzee, Luigi Nassimbeni and Hong Su
Department of Chemistry, University of Cape
Town, Rondebosch 7701, South Africa
Keywords : Clathrates Thermal Decomposition
Structure
We have synthesised a number of crystalline inclusion compounds consisting of bulky organic hosts and volatile guests. The compounds have been characterised by various analytical methods. The host:guest ratios were determined by Thermal Gravimetry (TG), the thermal stabilities were evaluated by Differential Scanning Calorimetry (DSC), and their structures were elucidated by single crystal diffraction methods.
Special equipment has been developed in order to measure the kinetcs of enclathration and of desolvation. This consists of a levitation balance which allows a host compound to be exposed to the vapour of the guest under controlled conditions of temperature and pressure. Phase changes which occur in the host-guest reaction were monitored by powder X-Ray diffraction. We have also constructed a Quartz Micro Balance (QMB) which allows us to make kinetic measurements on Host-Guest systems with minimal amounts of material ( 0.1-0.2 mg of host compound ).
The activation energies of several reactions have been
evaluated , and their trends have been rationalised in terms of
the topology of the stuctures. In particular we have studied the
kinetics of desolvation of the inclusion compound between the
host: trans-9,10-dihydroxy-9,10-diphenyl-9,10-dihydroanthracene
and cyclohexanone. These kinetic runs exhibited extended periods
of induction. We have studied this reaction under various
conditions and have noted a direct correlation between the
activation energy and the pre-exponential factor derived by the
Arrhenius equation.