Solid-State NMR Study of Guest Molecule Dynamics in 4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds

Deuterium nuclear magnetic resonance (NMR) powder spectra, deuterium spin−lattice relaxation times (T 1) and 13C CP/MAS NMR spectroscopy are used to investigate guest motion in 4-alkyl-tert-butylbenzene/thiourea inclusion compounds (alkyl = tert-butyl, isopropyl, and ethyl). Differential scanning calorimetry data indicate no solid−solid phase transitions for any of the three inclusion compounds in the temperature range −100 to +200 °C. Carbon-13 CP/MAS dipolar dephasing experiments indicate that the phenyl ring of all three guests reorient rapidly about the C1−C4 axis at room temperature. Deuterium T 1 data for the 1,4-di-tert-butylbenzene-d 18/thiourea (DTBB-d 18/TU) inclusion compound display two distinct mimina. Internal methyl rotation modulates 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 2H spectra of the DTBB-d 18/TU inclusion compound provide insight into the conformation of the methyl protons within the tert-butyl group. Deuterium NMR spectra indicate that the phenyl ring of the guest DTBB-d 4 in thiourea reorients between three positions in the 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 the rate of phenyl ring reorientation with that of tert-butyl motion and shows that the two motions within the same molecule are not correlated. The deuterium NMR spectra of the guest 4-isopropyl-d 6-tert-butylbenzene in thiourea (ITBB-d 6/TU) can be simulated using a model where six-site exchange of the isopropyl group modulates the line shape while the methyl rotation remains fast (k > 108 s-1). At the temperatures investigated, 2H 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. Similarly, the changes in the 2H spectra of the 4-ethyl-d 3-tert-butylbenzene/thiourea clathrate (ETBB-d 3/TU) indicate that the ethyl group also reorients between six sites in the host channel, superimposed by fast methyl rotation, which remains rapid (>108 s-1) on a lowering of temperature. Again 2H T 1's are being influenced by internal methyl rotation (E a = 11.6 (±0.5) kJ/mol) over the temperature range investigated. Final