SOLID-STATE NMR-STUDY OF GUEST MOLECULE-DYNAMICS IN 4-ALKYL-TERT-BUTYLBENZENE THIOUREA INCLUSION-COMPOUNDS/

Deuterium nuclear magnetic resonance (NMR) powder spectra, deuterium s pin-lattice relaxation times (T-1) and C-13 CP/MAS NMR spectroscopy ar e used to investigate guest motion in 4-alkyl-tert-butylbenzene/thiour ea inclusion compounds (alkyl = tert-butyl, isopropyl, and ethyl). Dif ferential scanning calorimetry data indicate no solid-solid phase tran sitions for any of the three inclusion compounds in the temperature ra nge -100 to +200 degrees C. Carbon-13 CP/MAS dipolar dephasing experim ents indicate that the phenyl ring of all three guests reorient rapidl y about the C-1-C-4 axis at room temperature. Deuterium T-1 data for t he 1,4-di-tert-butylbenzene-d(18)/thiourea (DTBB-d(18)/TU) inclusion c ompound display two distinct mimina. Internal methyl rotation modulate s T-1 in the higher temperature region, while tert-butyl reorientation affects T-1 at lower temperatures. Activation energies of 12.0 (+/-0. 5) kJ/mol and 11.3 (+/-0.4) kJ/mol, respectively, were determined. Low -temperature H-2 spectra of the DTBB-d(18)/TU inclusion compound provi de insight into the conformation of the methyl protons within the tert -butyl group. Deuterium NMR spectra indicate that the phenyl ring of t he guest DTBB-d(4) in thiourea reorients between three positions in th e host hexagonal channel. Distortions of the thiourea channel at lower temperatures affect the populations of the three sites. Deuterium T-1 data for the DTBB-d(4)/TU inclusion compound allows a comparison of t he rate of phenyl ring reorientation with that of tert-butyl motion an d shows that the two motions within the same molecule are not correlat ed. The deuterium NMR spectra of the guest 4-isopropyl-d(6)-tert-butyl benzene in thiourea (ITBB-d(6)/TU) can be simulated using a model wher e six-site exchange of the isopropyl group modulates the line shape wh ile the methyl rotation remains fast (k > 10(8) s(-1)). At the tempera tures investigated, H-2 spin-lattice relaxation times for ITBB-d(6)/TU are being influenced by internal methyl rotation within the isopropyl group. An activation energy of 13.1 (+/-0.5) kJ/mol was calculated. S imilarly, the changes in the H-2 spectra of the 4-ethyl-d(3)-tert-buty lbenzene/thiourea clathrate (ETBB-d(3)/TU) indicate that the ethyl gro up also reorients between six sites in the host channel, superimposed by fast methyl rotation, which remains rapid (>10(8) s(-1)) on a lower ing of temperature. Again H-2 T-1's are being influenced by internal m ethyl rotation (E-a = 11.6 (+/-0.5) kJ/mol) over the temperature range investigated. Finally, the rate of methyl rotation within each of the three functional groups is correlated with the strength of intramolec ular interactions within the respective alkyl groups.