Interpretation of electron diffraction data from organic microcrystals: a theoretical perspective.
The current framework for the quantitative analysis of transmission electron diffraction data from organic microcrystals is discussed. The appropriate formulation depends upon the crystal perfection: solution-grown material tends to be nearly perfect with regular elastic bends whilst the epitaxial samples are usually considerably less perfect with highly irregular bends. It is possible to obtain Bragg intensity data which are near-kinematical and thus amenable to ab initio structure analysis. Thereafter, the further refinement of the model proceeds with a consideration of dynamical and crystal bending effects. If both of these are small it is a good approximation to analyse them independently rather than use the correct description of dynamical scattering from a deformed foil. The analysis of three-dimensional data sets from tilt series is considered. Although the technique has been successfully demonstrated in several instances there are still details of the analysis that need further clarification.
Direct phasing of electron diffraction data from organic crystals: the effect of n‐beam dynamical scattering
The conditions under which direct phasing methods might be used to determine the crystallographic phases from electron diffraction intensity data are investigated through numerical calculations for two organic crystals. The Cowley-Moodie multislice formulation of dynamical diffraction theory is used to calculate the diffraction intensity data for various crystal thicknesses, at electron energies of 100 keV and 1.0 MeV. The direct phasing method is found to give generally correct phases up to a crystal thickness of about 75 A at 100 keV. The use of higher electron energy (i.e. 1.0 MeV) produces a significant improvement in the success of the direct phasing approach at crystal thicknesses greater than 75 A.
Calculation of the van der Waals potential energy for polyethylene and polytetrafluoroethylene as two-atom and three-atom chains: rotational freedom in the crystals
The van der Waals potential energy of polyethylene and polytetrafluoroethylene has been computed as a function of both two and three angles of rotation around the bonds of the backbone chain, the bond lengths and valence angles being held fixed. The calculations show that there are more minima than is predicted by the 'equivalence principle'. The implications of these results are important in the crystalline polymers and in the physical chemistry of the polymer solutions as well as in statistical-mechanical theories. In addition the evaluation of the van der Waals potential energy in the crystal of polytetrafluoroethylene by rotating in phase the polymer chains around their helical axes suggests that, surprisingly, the macromolecules are free-rotating.