A DEUTERIUM NMR-STUDY OF GUEST MOLECULAR-DYNAMICS IN TRIS(5-ACETYL-3-THIENYL)METHANE INCLUSION-COMPOUNDS

Deuterium nuclear magnetic resonance (NMR) spectra and spin-lattice re laxation times (T-1) are used to investigate the guest molecular dynam ics of tris(5-acetyl-3-thienyl) methane (TATM) inclusion compounds. Th e seven guests: acetonitrile, nitromethane, dimethyl sulfoxide, benzen e, mesitylene, ortho-xylene, and para-xylene show a wide variety of mo tional behaviour. The reorientation of acetonitrile in TATM and nitrom ethane in TATM were both modelled as precession on a cone, the base of which is more elliptical in shape for nitromethane. as would be expec ted considering their molecular symmetries. DMSO in TATM does not unde rgo any reorientation other than methyl rotation at the temperatures i nvestigated. At low temperatures, the H-2 lineshape and the deuterium spin-lattice relaxation time both depend on the rate of methyl rotatio n. Activation barriers of 11.7 (+/-0.4) kJ/mol and 11.2 (+/-0.5) kJ/mo l were found from the two techniques, respectively. Benzene undergoes sixfold reorientation about the principal molecular axis; however, the rate is still greater than 10(8) Hz down to 113 K. The spin-lattice r elaxation time profile does not reach a minimum on decreasing temperat ure to 112 K, while the slope of this plot provides an activation ener gy of 4.1 (+/-0.4) kJ/mol for the sixfold reorientation. The deuterium NMR spectra of mesitylene-d(9) in TATM can be simulated using a model where the guest occupies two different sites in the TATM lattice, wit h the guest molecules performing in-plane C-3 rotation either very rap idly (k > 10(8) Hz) or very slowly (k < 10(3) Hz), with the relative p opulations of each changing with temperature. Another model proposed s uggests the possibility that there is a continuous distribution of mot ional rates, the median of which is increasing with temperature. This second model is more realistic; however, too many parameters are prese nt to consider a detailed fit. Finally, both o-xylene and p-xylene are rigidly held in the TATM clathrate, while rotation of the methyl grou ps is rapid.