Structural and dynamic properties of the 1,10-dibromodecane/urea inclusion compound, investigated by variable-temperature powder X-ray diffraction, solid-state 2H NMR lineshape analysis and solid-state 2H NMR spin–lattice relaxation time measurements

Abstract

Powder X-ray diffraction and solid-state ²H NMR lineshape analysis and spin–lattice relaxation time measurements have been used to investigate structural and dynamic properties of the 1,10-dibromodecane/urea inclusion compound. This solid has the characteristic urea ‘host’ tunnel structure of conventional urea inclusion compounds and the 1,10-dibromodecane ‘guest’ molecules are located within these tunnels.

Variable-temperarture powder X-ray diffraction studies have demonstrated that a phase transition (at ca. 140–146 K) in 1,10-dibromodecane is associated with a change in the (average) symmetry of the urea tunnel structure from hexagonal (high-temperature phase) to orthorhombic (low-temperature phase) and is associated with only a minor structural distortion. From ²H NMR spectroscopy of [²H₂₀]1,10-dibromodecane/urea, the dynamic properties of the guest molecules are best described as rotational diffusion in a six-fold cosine potential, and there is no significant discontinuity in this dynamic process at the phase-transition temperature. The results suggest that the dynamic properties of all CD₂ groups in the guest molecule are indistinguishable (on the timescale of the ²H NMR technique). ²H NMR spectroscopy of 1,10-dibromodecane/[²H₄]urea indicates that, at sufficiently high temperature, the urea molecules undergo 180° jumps about their CO bonds. Activation parameters have been determined for the reorientational motions of the 1,10-dibromodecane and urea molecules.

The structural and dynamic properties of the 1,10-dibromodecane/urea inclusion compound, determined in this work, are compared in detail with the corresponding properties determined previously for alkane/urea inclusion compounds.