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 com pound have been investigated by Raman spectroscopy (between 90 K and r oom temperature) and IR spectroscopy (at room temperature). The intern al vibrational modes for the urea molecules in the 1,6-dibromohexane/u rea inclusion compound are similar to those for other urea inclusion c ompounds, consistent with the similar local structural properties of t he urea molecules in these structures. However, significant structural and conformational differences are observed between the 1,6-dibromohe xane/urea inclusion compound and other urea inclusion compounds. In pa rticular, the wavenumbers of the lattice modes reflect the different c rystallographic properties of these materials. In addition, the 1,6-di bromohexane molecule has two gauche end-groups in its urea inclusion c ompound while alpha,omega-CnH2nBr2 molecules in the urea inclusion com pound with n > 6 are essentially 'trans'. This material is therefore a model system for characterization of the vibrational properties of al pha,omega-dibromoalkane molecules in this conformation. Normal-mode ca lculations and force-field refinement are reported for the 1,6-dibromo hexane molecule with both end-groups gauche (point group C-i).