Chain End Dynamical and Conformational Properties of n-Nonadecane Molecules in Urea Inclusion Compounds: A Study by Deuterium NMR Spectroscopy

The 2 H NMR spectra of selectively deuterated n- nonadecane guest molecules CD 3 CD 2 (CH 2 ) 15 CD 2 CD 3 included within the channels of urea, CO(NH 2 ) 2, have been measured between 120 K and room temperature for two orientations of the applied magnetic field with respect to the single-crystal main axis (channel axis). The spectra reveal clearly the existence of a slow reorientational process (on the 2 H NMR time scale) of the whole chain in the low-temperature phase of the sample, while the chain ends perform fast torsional vibrations. On crossing the phase transition, the slow reorientational process transforms into fast uniaxial reorientations of the whole chains while a new fast dynamical process involving transitions between the tt, gt, and tg conformations is evidenced. Assuming that the amplitude of the fast torsional vibrations do not change abruptly on crossing the transition, we find an amount of gt conformers on the order of 5%, in excellent agreement with that obtained from other techniques. We also discuss and summarize the dynamical behavior of the guest n -nonadecane molecules in both the low-temperature and high-temperature phases of the urea inclusion compound, on the basis of the time windows probed by different spectroscopic techniques.