Radiotracer studies of self-diffusion in the plastic solids norbornylene and norbornane

Abstract

Radiotracer self-diffusion measurements have been made for norbornane (bicyclo[2,2,1]heptane) and norbornylene (bicyclo[2,2,1]hept-2-ene) in their h.c.p. plastic crystalline phases. In the case of norbornane the measurements were limited by the poor quality of the specimens; however, the results are consistent with existing n.m.r. data and support the view that self-diffusion occurs via a monovacancy mechanism. More extensive measurements were possible for norbornylene, and in the temperature range 251 to 308 K the atmospheric pressure results fit the equation D_t/m²s^–1=[2.6^+3.3_–1.5] 10^–5 exp (–49.2±2/kJ mol^–1//_RT). Measurements on oriented norbornylene specimens showed that any anisotropy of the self-diffusion was within the experimental error. Pressure effect studies for norbornylene at 281 and 301 K yielded an activation volume equal to 0.8 ± 0.1 times the molar volume. These results are in excellent agreement with existing n.m.r. data and confirm a monovacancy mechanism of self-diffusion. The present work suggests that in cases where there is a disagreement between radiotracer and n.m.r. measurements this cannot be explained simply in terms of the entropy of fusion of the material.