Crystal packing calculations and Rietveld refinement in elucidating the crystal structures of two modifications of 4-amidinoindanone guanylhydrazone

The crystal structures of two modifications (termed A and B) of 4-amidinoindanone guanylhydrazone (AIGH), 2-(4-amidino-1-indanylideneamino)guanidine, have been determined. Modification B, for which single crystals were available, has been treated in the usual manner. The determination of modification A, for which no single crystals were available, was made on the basis of the X-ray powder pattern and ab initio packing calculations. It has been shown that it is possible to determine in a routine manner the crystal structure of a polar and moderately flexible molecule, which can be specified as several tautomeric forms, using a combination of high-precision computational chemistry and Rietveld refinement. The resulting R-factor was ca 10%. Although the resolution of the powder diagram allowed for indexing, it is shown that the structure determination is also possible without indexing the powder diagram. The presented structure determination should be understood as an example of a new and generalized use of the Rietveld refinement where the two main problems, indexing of the powder pattern and making an initial structure guess, could be bypassed.

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