Deuterium NMR and x-ray crystallographic studies of ...
|Title||Deuterium NMR and x-ray crystallographic studies of guest and host motions in the thiourea/1, 4-di-tert-butylbenzene inclusion compound|
|Author(s)||G. H. Penner, James M. Polson, C. Stuart, G. Ferguson, B. Kaitner|
|Journal||Journal of Physical Chemistry|
|Abstract||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 H-2 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 H-2 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-temperature 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 x 10(3) and 2.0 x 10(6) 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 12.3 kJ/mol, respectively. Deuterium line-shape studies of the thiourea dynamics in TU-d4 and TU-d4/DTBB yield activation energies for 180-degrees flips about the C=S bond of 47 and 46 kJ/mol, respectively.|
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