U. Werner-zwanziger et al., Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit, APPL MAGN R, 17(2-3), 1999, pp. 265-281
H-2 NMR (nuclear magnetic resonance) spectroscopy, in conjunction with X-ra
y diffraction experiments, was used to characterize the guest motions of 1,
6-dibromohexane in its urea inclusion compound. These motions are character
ized by alkyl chain jumps between two conformations, each approximately gau
che to the terminal bromines, which remain stationary. In this distorted ur
ea channel, one conformer is heavily preferred, but thermally activated pop
ulation of the unfavorable conformer leads to reversible, temperature-depen
dent changes in the unit cell parameters. Although rapid motions of the gue
st chain give rise to ambiguities in the interpretation of the H-2 NMR spec
tra, fortuitous temperature-independent spectral features of guests contain
ing deuterium at the alpha, beta and gamma positions indicate that the gues
t motion resembles a two-site jump with unequal probabilities. Analytical l
ineshape calculations on the three sets of H-2 NMR spectra indicate a singl
e jump mechanism in which the range of jump angles is narrowly prescribed.
This NMR model provided a starting point for successful solution and refine
ment of the crystal structures at 213 and 298 K, which had been complicated
by motional disorder.