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

Abstract

2H NMR (nuclear magnetic resonance) spectroscopy, in conjunction with X-ray diffraction experiments, was used to characterize the guest motions of 1,6-dibromohexane in its urea inclusion compound. These motions are characterized by alkyl chain jumps between two conformations, each approximately gauche to the terminal bromines, which remain stationary. In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters. Although rapid motions of the guest chain give rise to ambiguities in the interpretation of the2H NMR spectra, fortuitous temperature-independent spectral features of guests containing deuterium at the α, β and γ positions indicate that the guest motion resembles a two-site jump with unequal probabilities. Analytical lineshape calculations on the three sets of2H NMR spectra indicate a single jump mechanism in which the range of jump angles is narrowly prescribed. This NMR model provided a starting point for successful solution and refinement of the crystal structures at 213 and 298 K, which had been complicated by motional disorder.