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

• Published in: The journal of physical chemistry. B Vol. 101; no. 44; pp. 9087 - 9097
• Main Authors: Sidhu, Paul S, Penner, Glenn H, Jeffrey, Kenneth R, Zhao, Baiyi, Wang, Zi Lin, Goh, Irene
• Format: Journal Article
• Language: English
• Published: American Chemical Society 30.10.1997
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp972128h

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. Finally, the rate of methyl rotation within each of the three functional groups is correlated with the strength of intramolecular interactions within the respective alkyl groups.