DEUTERON NMR-STUDIES OF GUEST MOTION IN ALKANOIC ACID UREA INCLUSION-COMPOUNDS

The orientation dependence of deuteron relaxation times of Zeeman (T-1 Z) and quadrupole (T-1Q) order have been measured in the temperature r ange -30 to +45 degrees C for polycrystalline urea inclusion compounds containing alkyl deuterated octanoic acid-d(18), dodecanoic acid-d(23 ), and hexadecanoic acid-d(31). These data were fitted to a model, bas ed on Redfield relaxation theory, which incorporates rapid rotation ab out the long molecular axis and slower, limited angle libration of ind ividual CD bonds about a perpendicular axis. The activation energies f or rotational diffusion about the long axis are 17.4 +/- 0.8 kJ/mol fo r octanoic acid-die, 14.4 +/- 0.6 kJ/mol for dodecanoic acid-d(23), an d 22.5 +/- 0.4 kJ/mol for hexadecanoic acid-d(31) guests. These values are larger than those for alkane guests with the same number of carbo n atoms, and they increase more rapidly with increasing chain length. Quadrupole coupling constants, determined at room temperature from mea surements on large single crystals, are consistent with a small gauche population near the methyl end of the alkanoic acid chains. Analytic models which relate the measured quadrupole coupling constants and rel axation times to conformational probabilities and dynamics require the assumption that rotations about individual C-C bonds are uncorrelated . Newtonian molecular dynamics simulations reveal that this incorrect assumption introduces significant error. (C) 1998 American Institute o f Physics.