ON THE MINIMUM NUMBER OF BEAMS NEEDED TO DISTINGUISH ENANTIOMORPHS INX-RAY AND ELECTRON-DIFFRACTION

The minimum strategy for distinguishing enantiomorphs by dynamical dif fraction is determined. It is found that, in the absence of anomalous dispersion, it is possible to determine the absolute hand of an enanti omorphic crystal by three-beam dynamical X-ray or electron diffraction in a general orientation only if a fourth noncoplanar reciprocal-latt ice point can be identified. Three-beam dynamical diffraction alone is unable to distinguish enantiomorphic forms. Identification is possibl e using four or more dynamical beams, in general, unless all relevant structure factors lie on a plane in reciprocal space passing through t he origin. Supporting computations are given.