Distance analysis of Patterson calculations and molecular mechanics as aids in structure solution

摘要

Two methods (SUPSYMM and MMXRAY) for crystal structures insoluble through normal crystallography have been developed. SUPSYMM needs only a list of Patterson superposition peaks as its starting point. Using distance analysis and vector projections, symmetry relations between the superposition peaks are identified which are exploited to identify probable peak positions for least-squares refinement. MMXRAY solves large organometallic structures by using Patterson superposition and molecular mechanics. Patterson superposition techniques can usually locate at least the heavy atoms. Molecular mechanics is used to model the remaining organic parts of the structure. MMXRAY converts the modeled structure to the original crystallographic reference system. Two programs designed to teach the basics of both powder and single crystal X-ray diffraction methods to undergraduate physical chemistry students are presented in the Appendix. Several crystal structures solved through routine crystallographic routes are also discussed.