Application of Rietveld Refinement Combined with Force Field Energy Minimization to Structure Investigation of Cyclo-tris(2.6-pyridyl formamidine)

The structure refinement of X-ray powder data using the Rietveld method often results in false minima, i.e. not necessarily the energetically favored result. By introducing force fields as a new type of constraint into the recently developed BGMN program, a fully featured X-ray Rietveld system was created. With the model of bonding and nonbonding interactions, it was finally possible to refine the crystal structure of an aromatic trimer. By means of simple powder diffraction measurements, a set of structural data was obtained which was fitted mathematically to a structural model containing the geometry optimization by ab initio calculations, the indexing of diffraction pattern, the search of reasonable space group and the structure refinement under force field constraints. As an example, it was found that cycle-tris(2,6-pyridyl formamidine) crystallizes in a monoclinic unit cell with a = 28.17 A, b = 14.68 A, c = 4.43 A, 8 = 90.04 deg. The symmetry was determined as P2, /c, unique axis b, cell choice 3, Z = 4. The expected threefold symmetry of the cyclic compound was slightly disturbed. The best approximation was obtained when water molecules were packed additionally into the unit cell. Copyright (C) JCPDS-International Centre for Diffraction Data 1999