Solid-State NMR Study of Guest Molecule Dynamics in 4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds Paul S. Sidhu, † Glenn H. Penner,* ,† Kenneth R. Jeffrey, ‡ Baiyi Zhao, † Zi Lin Wang, † and Irene Goh † Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1, and Guelph-Waterloo Program for Graduate Work in Physics, Department of Physics, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1 ReceiVed: July 1, 1997 X Deuterium nuclear magnetic resonance (NMR) powder spectra, deuterium spin-lattice relaxation times (T 1 ) and 13 C 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 C 1 -C 4 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 2 H 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 > 10 8 s -1 ). At the temperatures investigated, 2 H 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 2 H 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 (>10 8 s -1 ) on a lowering of temperature. Again 2 H 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. Introduction Thiourea is known to form channel inclusion compounds with a wide variety of organic guest molecules, such as cyclohexane derivatives, branched-chain paraffins, and some alkyl-substituted aromatic compounds. 1,2 In the presence of guest molecules, the thiourea lattice usually adopts a rhombohedral structure. 3-5 X-ray diffraction studies have shown that the thiourea molecules form a hexagonal channel, of approximately 7.0 Å diameter, in which guests of appropriate size reside. 6,7 The lattice is held together by hydrogen-bonded thiourea molecules. Calorimetric studies of thiourea clathrates often show phase transitions, which are associated with a change in the channel’s size or shape. 8 The thiourea host has a repeating structure in that the thiourea molecules point directly into the center of the channel at every third level, and it is usually at this point where the guest molecule is located. 9 A prerequisite for the understanding of the stability of these inclusion compounds is a knowledge of the dynamic behavior of the guest molecule. X-ray diffraction studies of thiourea clathrates are often unable to locate the atomic positions of the guest molecule, but rather show a smeared-out electron density resulting from fast reorientational motion of the guest. 10 Deuterium NMR studies of the guest molecular dynamics in thiourea inclusion compounds of cyclohexane, 11 ferrocene, 12 and 1,4-di-tert-butylbenzene 13 have recently been reported. We have chosen to continue studies of the 1,4-di-tert-butylbenene/thiourea (DTBB/TU) inclusion compound (1),in addition to examining the 4-isopropyl-tert-butylbenzene/thiourea (ITBB/TU) inclusion compound deuterated exclusively on the isopropyl methyl † Guelph-Waterloo Centre for Graduate Work in Chemistry. ‡ Guelph-Waterloo Program for Graduate Work in Physics. X Abstract published in AdVance ACS Abstracts, October 1, 1997. 1 9087 J. Phys. Chem. B 1997, 101, 9087-9097 S1089-5647(97)02128-7 CCC: $14.00 © 1997 American Chemical Society