AUTOMATED PHASE DETERMINATION IN ELECTRON CRYSTALLOGRAPHY - THERMOTROPIC PHASES OF THIOUREA

By use of the tangent formula in the computer program QTAN, it is possible to determine the crystal structures of two thermotropic phases of thiourea from texture electron diffraction intensity data published originally by Dvoryankin and Vainshtein. There is no intervention in this phasing procedure after origin definition and the correct structure is identified from the multiple listing of solutions (via permutation of phase ambiguities) by the lowest value of NQUEST, as it also would be in X-ray crystallography. The procedure is useful for centrosymmetric or noncentrosymmetric structures. Although the initial crystal structure solutions are quite similar to independent X-ray determinations, it is difficult to refine them to chemically accurate molecular architectures when the data are distorted by dynamical scattering. Hence, the non-randomness of multiple beam dynamical scattering seems to preserve crystallographic phase information in the distorted intensities even if these intensities are not useful for rigorous refinements.