Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit

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.