Competition between hydrogen bonding and arene-perfluoroarene stacking : X-Ray diffraction and molecular simulation on 5,6,7,8-tetrafluoro-2-naphthoic acid and 5,6,7,8-tetrafluoro-2-naphthamide

Single crystal X-ray diffraction and PIXEL calculation of intermolecular pair energies, with partitioning into coulombic and dispersive factors, have been performed on the title crystals with the aim of testing the relative efficiency of molecular recognition synthons. X-Ray diffraction reveals that the crystals contain extended hydrogen-bonded layers, with various degrees of aromatic ring overlap between the layers. The calculation of molecular pair energies quantitatively reveals that the primary binding force in these crystals is hydrogen bonding, and that the arene-perfluoroarene stacking mode becomes less important, or almost optional. The various degrees of offset between overlapping aromatic rings found in the present crystals, also in comparison with some non-hydrogen bonding analogs, suggest a transition from perfect, highly stabilizing Ar H-Ar F overlap, to the less favourable, but still energetically stabilizing, Ar F-Ar F and Ar H-Ar H overlaps. The selectivity towards Ar H-Ar F stacking is not overwhelming.