Characterization of gauche end-groups in alpha,omega-dibromoalkanes: Vibrational properties of the 1,6-dibromohexane/urea inclusion compound

The vibrational properties of the 1,6-dibromohexane/urea inclusion compound have been investigated by Raman spectroscopy (between 90 K and room temperature) and IR spectroscopy (at room temperature). The internal vibrational modes for the urea molecules in the 1,6-dibromohexane/urea inclusion compound are similar to those for other urea inclusion compounds, consistent with the similar local structural properties of the urea molecules in these structures. However, significant structural and conformational differences are observed between the 1,6-dibromohexane/urea inclusion compound and other urea inclusion compounds. In particular, the wavenumbers of the lattice modes reflect the different crystallographic properties of these materials. In addition, the 1,6-dibromohexane molecule has two gauche end-groups in its urea inclusion compound while alpha,omega-CnH2nBr2 molecules in the urea inclusion compound with n > 6 are essentially 'trans'. This material is therefore a model system for characterization of the vibrational properties of alpha,omega-dibromoalkane molecules in this conformation. Normal-mode calculations and force-field refinement are reported for the 1,6-dibromohexane molecule with both end-groups gauche (point group C-i).