Self-Diffusion in Some Plastic Crystals

Abstract

Self-diffusion phenomena in several plastic crystals were studied by use of high-resolution n.m.r. techniques which have been hither to applied to pure liquid or solution systems. The pursuit of spectral narrowing processes, with the aid of Kubo-Tomita formula, makes it possible to evaluate the kinetic aspects of diffusional motion. Activation enthalpies for diffusion were obtained for trimethylacetic acid, triethylenediamine, succinonitrile and ammonium nitrate; they were 8.09 ± 0.25, 21.7 ± 1.0, 10.7 ± 0.5 and 11.3 ± 0.5 kcal/mole, respectively. These data, as well as those available for the other plastic crystals, were correlated with the latent heat of fusion in a similar manner to those for cubic metals. Also, the chemical shift of OH resonance relative to CH₃ resonance line of trimethylacetic acid was measured for the crystal and liquid phases. Temperature coefficient of OH chemical shift is of the order of 1.0 × 10⁻² ppm/°C. This observation and the other evidence suggest strongly the validity of the model, by which Muller and Reiter have explained the main origin of temperature variation of OH chemical shift in some hydrogen-bonded systems.