Crystal structures of twisted di(2-pyridyl)ketone and planar azo-di(2-pyridine) and their discussion based on PM3 enthalpy hypersurfaces for the isoelectronic molecules

Single crystal structure determinations prove the two pyridine substituents in di(2-pyridyl)ketone (H4C4NC)(2)C=O to be twisted out of the carbonyl skeleton plane by torsion angles omega (OCCN) of 41 degrees and -163 degrees, in contrast to their planar arrangement in azo-di(2-pyridine) (H4C4NC)-N=N-(CNC4H4). In order to rationalize the surprising difference between the two isoelectronic molecules, approximate PM3 enthalpy of formation hypersurfaces have been calculated for each of the two ring torsions, which are assumed to be the dominant ones among the 3N - 6 = 60 degrees of freedom. For both the ketone and the azo derivative, global minima are calculated, the torsion angles of which deviate from the crystal structure results, and, therefore, support the assumption that both the experimentally determined twisting of di(2-pyridyl)ketone as well as the flattening of azo-di(2-pyridine) might be affected by the crystal packing.