A spectroscopic study of the structure and dynamics of the urea inclusion adduct of stearic acid

Abstract
The urea inclusion adduct of stearic acid and derivatives deuterated at positions 2, 4, 12, 16, and 18 have been prepared. The temperature dependence of the Raman, infrared, and 2H NMR spectra reveals the existence of a phase transition centered at 234 K. The distortion of the urea host is completed within a 4 K interval while the order–disorder transition of the included stearic acid occurs over 30 K. The changes in motional freedom of the included chain are accompanied by a change in the geometry of the dimeric carboxyl moiety. Chain motion is not completely frozen in the low-temperature crystal form and evidence is presented for position-dependent off axis and torsional motions.

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