Deuterium NMR and X-ray Crystallographic Studies of Guest and Host Motions in the Thiourea/1,4-Di-tert-butylbenzene Inclusion Compound

Article abstract of DOI:10.1021/j100191a068

Deuterium nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation times are used to investigate the guest and host molecular dynamics of solid 1,4-di-tert-butylbenzene-d4 (DTBB-d4), 1,4-di-tert-butylbenzene-d18 (DTBB-d18), the thiourea/1,4-di-tert-butylbenzene-d4 inclusion compound (TU/DTBB-d4), the thiourea/1,4-di-tert-butylbenzene-d22 inclusion compound (TU/DTBB-d22), the thiourea-d4/1,4-di-tert-butylbenzene inclusion compound (TU-d4/DTBB), and thiourea-d4 (TU-d4).X-ray crystallographic studies of TU/DTBB-d4 have been carried out at 291 K.In solid DTBB the phenyl ring is essentially static whereas the tert-butyl groups are undergoing rapid reorientation of both methyl and tert-butyl groups.Attempts to analyze the (2)H spectra and T1 data for DTBB-d18 suggest that the dynamics of the methyl and tert-butyl groups are nearly equivalent, and as a result, a satisfactory analysis, yielding methyl and tert-butyl rotational activation energies, was not possible.X-ray diffraction results for TU/DTBB-d4 suggest that, at 291 K, the phenyl ring is occupying three nearly equivalent sites.The (2)H NMR line shapes between 186 and 392 K were interpreted using a model in which the phenyl ring is rapidly flipping between three positions, with one position less favored.At 296 and 186 K the populations are 0.81 : 1.00 : 1.00 and 0.20 : 1.00 : 1.00, respectively.Relaxation times obtained between 111 and 322 K show no minimum, supporting the assumption of very rapid phenyl ring reorientation.For TU/DTBB-d22 a high-tmperature T1 minimum is well-defined, and a second minimum, corresponding to tert-butyl group rotation, is reached at the lowest attainable temperatures.Line-shape simulations of the spectrum at 77 K yield methyl and tert-butyl group rotational rates of 1.0 * 10³ and 2.0 * 10⁶ s^-1, respectively.Analysis of the higher temperature spectra (109-172 K) and T1 data (167-300 K) yield methyl rotation activation energies of 12.7 and 2.3 kJ/mol, respectively.Deuterium line-shape studies of the thiourea dynamics in TU-d4 and TU-d4/DTBB yield activation energies for 180 deg flips about the C=S bond of 47 and 46 kJ/mol, respectively.