Crystal Structures of Fluorinated Pyridines from Geometrical and Energetic Perspectives

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.