EXAFS spectroscopic studies of the bromine environment in the crystalline inclusion compounds formed between urea and α,ω-dibromoalkanes

Abstract

EXAFS spectroscopy of the bromine K-edge has been used to investigate local structural properties of urea inclusion compounds containing α,ω-dibromoalkane ‘guest’ molecules [Br(CH₂)_nBr; n= 6–11]. In these inclusion compounds, the α,ω-dibromoalkane molecules are included within a urea ‘host’ substructure containing linear, parallel tunnels. For the inclusion compounds containing Br(CH₂)_nBr ‘guest’ molecules with n= 7–11, the host substructure is that of the conventional urea inclusion compounds, whereas the inclusion compound containing Br(CH₂)₆Br as the guest has a different host substructure.

The principal aim of these studies was to determine the Br⋯Br distance between adjacent guest molecules in the urea tunnel. For the inclusion compounds containing Br(CH₂)_nBr molecules with n= 7–11, however, an accurate determination of the Br⋯Br distance has not been possible from data collected at room temperature and at 77 K (this was also found for data collected at 9 K for 1,10-dibromodecane/urea), probably because of dynamic disorder at high temperature and static positional disorder at low temperature. On the other hand, backscattering from the first three intramolecular carbon neighbours on the Br(CH₂)_nBr molecule is detected in the Br EXAFS, and structural parameters for these neighbours have been determined. In these inclusion compounds, the periodicities of the host and guest substructures along the tunnel axis are incommensurate, and therefore each guest molecule within a given tunnel is in a different environment with respect to the host substructure. As a consequence, no well defined features in the Br EXAFS spectra arising from backscattering by atoms in the host substructure are expected or observed.

In the 1,6-dibromohexane/urea inclusion compound, the guest substructure is commensurate with the host substructure. The Br K-edge EXAFS results for 1,6-dibromohexane/urea are consistent with known structural features of this inclusion compound.