Enantiomorph identification of crystals belonging to the point groups of 622 and 6 by convergent-beam electron diffraction method

A CBED method proposed by the present authors for chiral identification of enantiomorphic crystals has been successfully applied to crystals with the point groups of 622 and 6. The intensity asymmetry of ZOLZ and/or FOLZ reflections of the Bijvoet pairs is utilized for enantiomorph identification and such intensity asymmetry is noted regardless of beam convergence angle but the extent to which the asymmetry is noted depends on crystal thickness and accelerating voltage. The crystal thickness should be smaller than the extinction distance of relevant reflections that constitute 'umweganregung' in multiple scattering, which is responsible for the intensity asymmetry. As a result, the maximum crystal thickness, below which the asymmetric intensity distribution of Bijvoet reflections is evident, increases as the accelerating voltage is increased. When the crystal thickness is small, however, the intensity asymmetry is more pronounced as the accelerating voltage is decreased due to the increased propensity of multiple scattering.