CONFORMATION AND STABILITY OF ALKANE/UREA INCLUSION-COMPOUNDS - EVIDENCE FROM DEUTERIUM NMR-SPECTROSCOPY

n-Alkanes contained as guests in urea inclusion compounds (UIC) and once thought to exist exclusively in the all-trans conformation have recently been proposed to contain relatively large amounts of gauche conformers. Those suggestions have subsequently been challenged in a recent Letter to this Journal by Casal, who suggested that the observed gauche conformations were due to free alkane formed by decomposition of the UIC's. Deuterium quadrupole echo NMR spectra have now been obtained of the urea inclusion compounds containing the normal alkanes with n = 8, 10, 12, 16, 19, and 36, samples of which were subjected to magic angle spinning or ultrasonic agitation. The deuterium spectra show that while the C-8 and C-10 UIC's are indeed unstable at the temperatures prevailing during these treatments, their decomposition does not affect conclusions drawn from the C-13 NMR spectra. The deuterium quadrupolar splittings reported here are most readily explained in terms of trans-gauche isomerization near the ends of the n-alkanes, but the gauche content is less than that suggested by MM2 and MD calculations.