Electron crystallography of the polymethylene chain. 3. Three-dimensional crystal structure of a refined paraffin wax 1

Electron crystallography of the polymethylene chain. 5. Three-dimensional structure of a Fischer-Tropsch wax

The crystal structure of a Fischer-Tropsch wax, Shell Callista 158, has bee determined in three dimensions from electron diffractio intensity data. Although there is an orthorhombic layer arrangement with apparent dimensions, a = 7.42, b = 4.96, c = 94.50 Å, true lamellae cannot form, due to a ‘nematocrystalline’ arrangement of very long chain components,generating a continuous chain direction through adjacent chain layer clusters. The structure is actually a superlattice of the Pnam cell observed for high molecular weight polyethylene.

Electron crystallography of the polymethylene chain. 4. Defect distribution in lamellar interfaces of paraffin solid solutions

As is also found for paraffin lamellar crystals heated near the rotator or melt transition, the major change in electron diffraction patterns imposed by lamellar disorder in binary or multicomponent solid solutions is the attenuation and intensity modification of low-angle (00l) reflections. Using a Gaussian model for atomic occupancies near the interface, a systematic basis is found for the interfacial disorder thickness, describing the phenomenological ‘void’ distribution at the lamellar interface. (Vibrational spectroscopy has shown that this is actually a distribution of non-planar chain conformations.) Single crystal electron diffraction data from various co-soluble paraffin chain assemblies, used previously to determine their crystal structures (but also including a new determination for n-C36H74/ n-C38H78/ n-C40H82 1: 1: 1), are re-analyzed so that further distinctions can be made among simple binary and ternary solid solutions, multicomponent waxes, and low molecular weight polyethylene. Expressions of the latter polyethylene structure, involving ‘bridging molecules’ can be found in certain natural insect and plant waxes.

Electron crystallography of organic materials

The application of electron crystallography to the study of organic materials is reviewed, mainly in context of the author's own experience. Direct methods for crystallographic phase determination have been shown to be very effective for ab initio structure analyses with electron diffraction intensities, permitting the elucidation of previously uncharacterized crystal structures. Fruitful applications areas have included chain-folded linear polymers, pigments, polydisperse linear chain arrays and, surprisingly, the subgroup assembly of certain proteins.