Structural properties of α,ω-dibromoalkane/urea inclusion compounds: a new type of interchannel guest molecule ordering

Abstract

Structural properties of urea inclusion compounds containing α,ω-dibromoalkanes [Br(CH₂)_nBr; n= 7–10] as the guest component are reported. In these inclusion compounds, the host (urea) structure contains linear, parallel, non-intersecting channels (tunnels), within which the guest molecules are located. The structural properties of the guest molecules have been determined at room temperature using single-crystal X-ray diffraction techniques (primarily photographic methods), and represent the main focus of this paper.

Rationalization of the complete X-ray diffraction pattern from an α,ω-dibromoalkane/urea inclusion compound requires more than one three-dimensionally periodic reciprocal lattice; the interpretation of the complete diffraction pattern is discussed in detail for 1,10-dibromodecane/urea. For each α,ω-dibromoalkane/urea single crystal, the guest molecules are closely packed along the channels with a periodic repeat distance (|c_g|) that is incommensurate with the periodic repeat distance (|c_h|) of the host along the channel axis. In some regions of the crystal the guest molecules are ordered only along the channel axis whereas in other regions the guest molecules are ordered in three dimensions. In this three-dimensionally ordered structure, the offset (denoted Δ_g) between the ‘heights’ of guest molecules in adjacent channels is given by the exact relationship Δ_g=|c_g|/3. Thus, Δ_g depends on the value of |c_g|, and hence depends on the ‘length’ of the guest molecule. This represents a new mode of interchannel ordering of guest molecules in urea inclusion compounds. Within each single crystal, two domains of the guest structure can be distinguished by X-ray diffraction. Each domain has rhombohedral symmetry; the lattices that define the periodicities of the two domains differ in orientation [related by (2n+ 1)π/3 rotation about the channel axis; n= integer], but are otherwise identical. These two domains correspond to the observe and reverse settings of the rhombohedral average guest structure. Finally, the results are discussed in relation to the contrasting structural properties of urea inclusion compounds containing other types of guest molecules.