Transmission Electron Diffraction Intensities from Real Organic Crystals: Thin Plate Microcrystals of Paraffinic Compounds

Abstract

Solvent-grown microcrystals of long polymethylene compounds packing in the 0 ± methylene subcell are found to have some of the defect structures seen for crystalline polyethylene. Crystals composed either of rectangular layers or of oblique layers both exhibit the growth around a screw dislocation characterized earlier by other workers. In addition, rectangular layer crystals o f orthorhombic n-hexatriacontane give evidence of mfralamellar edge dislocations. When rectangular layer crystals of cetyl palm itate are grown from hot ethanol, the same surface corrugations are found as seen earlier for polyethylene and orthorhombic paraffins. Oblique layer crystals have not demonstrated these latter two defect structures. All crystals show evidence o f appreciable bending.As was found to be true for rectangular layers of n-paraffins, bending has some influence on the electron diffraction intensities from an oblique layer. Another important factor is a perturbation due to defects along long chain axes which disrupts the ordered packing of polar regions, and causes incoherent scatter from these regions. In addition, observed screw dislocations give a nonuniform end plane, causing isolation of layers from one another. Diffraction models accounting for bending or isolation o f chain layers in an oblique layer both explain the observed near in variance of certain intensities which are due to the coherent scattering from the line grating formed by rows o f carbon atoms in the tilted chain aggregate. Forbidden reflections predicted by a bending model are less important than those anticipated by the multiple scattering from a laminated layer model. As these forbidden reflections are mostly identified for multilayer crystals and since crystal structure analysis indicates most of the long chain in a oblique monolayer to be diffracting coherently, the concentration of Reneker-type defects in the centers of chains is probably not great enough to isolate small chain length laminae within a monolayer.

Electron crystal structure analysis of small organic molecules

“…However, the intensity distribution in the pattern is clearly a function of crystal bending (Dorset, 1980). Diffraction patterns from inclined paraffin chain structures show intensity changes more drastically altered by bending and also changes in zonal symmetry (Dorset, 1978a). These effects are also well explained by the model.…”

“…Even crystal bending of monomolecular layers will appear to originate from Patterson functions (8) different from the autocorrelation of the true crystal structure (0 with some reflections varying greatly with amount of crystal bend. The apparent crystal symmetry suggested by the diffraction pattern is not that of the unit cell (see Dorset, 1978a , 1984) to reveal the orientation of the complete long chain in the unit cell.…”

“…Second, is this lattice repeat along c (i.e., normal to the crystal face) small enough to ensure determination of a total crystal structure from an elastically deformed crystal? As shown above, this can be a major problem for many molec-ular crystals, and if the compounds in question (alkane derivatives, lipids) have a polymethylene subcell with a short spacing (2.54 A), this feature will dominate the diffraction pattern (Dorset, 1978a(Dorset, , 1980. Although favorable answers can be given to these questions for many linear polymer crystals (Moss and Dorset, 1982a1, use of e.d.…”

Interpretation of electron diffraction data from organic microcrystals: A theoretical perspective

“…Patterson peaks separated by large distances in the electron beam direction (i.e., large z,) will not contribute significantly to the diffracted beam intensities I(s), causing a loss of coherence in the electron beam direction. The resultant diffraction pattern will not necessarily represent the true zonal symmetry (Dorset, 1978) and unit cell contents (Dorset, 1980).…”