The results of the fifth blind test of crystal structure prediction, which show important success with more challenging large and flexible molecules, are presented and discussed.

No abstract

No abstract

An X-ray diffraction study of three bicyclic aziridines (2,2-dimethyl-4,6-diaryl-1,3-diazabicyclo-[3.1.0]hex-3-enes) reveals the absence of strong specific intermolecular interactions in the crystals. Therefore, crystal packing has been analyzed on the basis of quantum-chemical calculations of energy of intermolecular interactions between basic molecules located in the asymmetric part of the unit cell and molecules belonging to its first coordination sphere in the crystal. The analysis of the energy of intermolecular interactions indicates that the crystal structures of bicyclic aziridines are layered. The total energy of the interactions between the basic molecule and other molecules belonging to the layer is significantly higher as compared to the energy of the interactions with molecules of two neighbouring layers.

The low melting compounds 3-fluoropyridine, 3,5-difluoropyridine, 2,3,5-trifluoropyridine, and 2,3,5,6-tetrafluoropyridine were crystallized in situ on the diffractometer and analyzed by X-ray diffraction. The comparison with pentafluoropyridine shows a consecutive dependence of the arrangement of pyridine molecules with the increasing fluorine substitution. Starting with the monosubstituted representative, the crystal packing changes stepwise from herringbone packing to parallel arrangement of the molecules in the trifluorinated pyridine and then switches back to the edge-to-face form by the perfluorinated compound. To get a deeper insight into the aggregation behavior of the flourinated pyridines, the crystal packing motives were also analyzed on the basis of ab initio quantum-chemical calculations of the intermolecular interaction energy, using the MP2/6-311G(d,p) method. This approach allows the indication of energetically rich fragments in the crystal structures, called basic structural motifs, and the very weak attractive or rather repulsive nature of F···F interactions.