DYNAMIC PROPERTIES OF THE UREA MOLECULES IN ALPHA,OMEGA-DIBROMOALKANE/UREA INCLUSION-COMPOUNDS INVESTIGATED BY H-2 NMR-SPECTROSCOPY

H-2 NMR spectroscopy has been used to investigate the dynamic properties of the urea molecules in Br(CH2)(n)Br/[H-2(4)]-urea inclusion compounds (n =7-10) and in pure crystalline [H-2(4)]urea. In the Br(CH2]Br-n/urea inclusion compounds, the urea molecules form an extensively hydrogen-bonded host structure, which contains linear, parallel tunnels within which the Br(CH2)(n)Br guest molecules are located. The urea molecules undergo a two-site 180 degrees jump motion about their C=O axes, on the H-2 NMR timescale, at temperatures greater than ca. 200 K for the Br(CH2)nBr/[H-2(4)]urea inclusion compounds and at temperatures greater than ca. 300 K for pure crystalline [H-2(4)]urea. Quantitative details relating to the dynamic properties of the urea molecules in these solids are presented.