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, llheptane) and norbornylene (bicycIo[2,2,l]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 possibJe for norbornylcne, and in the temperature range 251 to 308 K the atmospheric pressure results fit the equation 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 0.840.1 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.

On self-diffusion in plastic crystals

“…measurements have also been reported for hexamethyldisilane (HMDS) [21], norbornane (NBA) [16,22] and adamantane [23,24]. We have not included these in table 2 although the N.M.R.…”

“…values and this effect increases as the melting point is approached. A comprehensive pressure dependence study of diffusion [28] has indicated that a divacancy mechanism may (HME) [15,26], hexamethyldisilane (HMDS) [21], norbornylene (NBE) [16,22], and norbornane (NBA) inset [16,22] plotted as a function of reciprocal temperature. The open data points are radio-tracer measurements.…”

The evidence for a metastable phase in norbornadiene: X-ray and quasi-elastic neutron scattering studies

“…Conversely, close-packing considerations allow one to imagine a displacement of the centre of mass when the molecule passes from one orientation to another. This hypothesis is strengthened by the existence of a long-range self-diffusion of the molecules, evidenced by NMR (Folland et a1 1973a, b, Chadwick andForrest 1978). The delocalisation of the molecules could also explain the ability of norbornadiene to crystallise at the same temperature either in an HCP lattice or in an FCC lattice.…”

“…Below Tt = 202 K the low-temperature phase is ordered. The molecular disorder in the plastic phase was evidenced by nuclear magnetic resonance (NMR) (Folland et a1 1973a, b, Chadwick andForrest 1978). But it turns out that the relaxation is mainly influenced by the selfdiffusion of the molecules, thus preventing precise information being obtained about their rotational dynamics.…”