H-2 NMR INVESTIGATIONS OF THE HEXADECANE UREA INCLUSION COMPOUND/

The molecular behavior of selectively deuterated n-hexadecane in the u rea inclusion compound is studied between 100 and 290 K by means of dy namic H-2 NMR spectroscopy employing line shape studies and spin-spin and spin-lattice relaxation experiments. Particular emphasis is given to the changes of the guest molecule behavior close to a solid-solid p hase transition occurring at lower temperatures which is accompanied b y a distortion of the urea lattice; It is demonstrated that spin-latti ce relaxation experiments are of particular help for the evaluation of the chain dynamics in such systems. A comprehensive computer analysis of the available experimental data could provide a detailed picture o f the hexadecane chains in the low-temperature phase and above the sol id-solid phase transition. In the low-temperature phase the alkyl chai ns are found to undergo fast but restricted rotational motions (rate c onstant ca. 10(7) s(-1)). In addition, intramolecular trans-gauche iso merizations and methyl group rotation contribute to the spin relaxatio n of the alkane chain ends. In the high-temperature phase the alkyl ch ains rotate rapidly (rate constant ca. 10(10) s(-1)) and almost unrest ricted around the channel long axis, giving rise to a dynamic (rotatio nal) disorder of the embedded alkyl chains. It therefore is concluded that the phase transition can be assigned to a (dynamic) order-disorde r transition. The alkane chains furthermore are found to be perfectly aligned with respect to the urea channel long axis. At the same time, they exhibit an internal flexibility gradient toward the chain ends. T he conformational order significantly is altered at the phase transiti on. Thus, the trans population in the low-temperature phase is given b y p(t) = 0.7 while in the high-temperature phase a value of p(t) = 0.9 5 has beer! derived.