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
Date 1992
Volume 96
Issue 12
Start page 5121
End page 5129
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:, 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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