Hydrogen-2 nuclear magnetic resonance observations on conformation and dynamics of alkyl chains trapped within solid urea inclusion channels

We report on a ²H NMR study of several selectively deuterated stearic acids and α-deuterated n-bromooctane-d₂ acting as guests in urea adducts. The stearic acid chain assumes an extended all-trans configuration along the channel axis at all temperatures; it is, except for the somewhat flexible end segment, immobile up to 140 K, when onset of discrete jumps of threefold symmetry about the channel axis was detected. The end-methyl dynamics is more complex, with rapid local fluctuations superimposed on the molecular jumps. We found this combined mode to be predominantly diffusive. Bromooctane-d₂ labeled at the α position was found to diffuse continuously about an axis perpendicular to the C-D bond, in accord with an extended all-trans chain conformation. With the shorter chain, onset of mobility is shifted to lower temperatures.