MOLECULAR-DIFFUSION AND PREMELTING IN HYDROGEN-BONDED ORGANIC-SOLIDS

The H-1 and C-13 NMR spectra of monocrystalline as well as powder samp les of 4-n-pentylcyclohexane acetic acid (PCA) were investigated close to their melting point T-m = 329 K. Temperature dependencies of the s pin-lattice relaxation times T-1 of the -COOH and -CH2- groups were me asured in this region. It was found that the monocrystalline and powde r samples have very similar T-1 values close to T-m, but the values st art to diverge at about 30 K below T-m. The activation energies (E(a)) have been calculated using the temperature dependencies of the integr ated intensities (I) of the narrow NMR signals and interpreted as the corresponding E(a) values for subgrain-boundary self-diffusion (E(a) = 58 kJ mol(-1), powder sample) and vacancy induced diffusion (E(a) = 1 70 kJ mol(-1), monocrystal). The results of earlier NMR studies of sol id carboxylic acids have been re-examined and the validity of the powe r law I similar to \T - T-m\(-gamma) has been checked. Values of gamma between 1 and 2 have been obtained for all substances except octadeca noic acid and PCA (monocrystal), which have larger values (4.2 and 2.9 , respectively).