Solid-state NMR investigations on urea inclusion compounds: Order and dynamics of 1,6-dibromohexane

The molecular properties of 1,6-dibromohexane in its urea inclusion compound are investigated by means of a multinuclear solid-state NMR spectroscopy. 13C CP/MAS and 1H MAS NMR studies (line shapes, spin–lattice relaxation measurements) were performed for the first time to probe the guest dynamics and conformational order. Variable temperature 2H NMR studies comprising line shape analysis as well as spin–lattice relaxation (T1Z, T1Q) experiments were done on samples with guest molecules selectively deuterated at two different positions. A quantitative analysis of the experimental data proves that the guest dynamics are dominated by mutual exchange between two gauche conformers. It is shown that these guest motions unequivocally can be quantified (type and time-scale) by a comprehensive analysis of the T1Z and T1Q data. In addition, there is evidence that other motional contributions, such as overall molecular fluctuations and lateral motions, also contribute to spin relaxation. The molecular behaviour of 1,6-dibromohexane in urea is completely different from that reported for the long chain analogues or for n-alkanes where typically unhindered overall rotational motions of the guests in their all-trans conformation around the urea channel long axis are discussed. The differences in guest ordering and dynamics are a direct consequence of the differences of the urea lattice structures (monoclinic vs. hexagonal urea lattice).