PACKING ANALYSIS OF ORGANIC-CRYSTALS CONTAINING C=O OR C=N GROUPS

Eighty crystal structures for organic molecules containing one C=O group, and 13 crystal structures for molecules containing one C=N group, with up to 20 non-hydrogen atoms, have been retrieved from the Cambridge Structural Database. The effects of the introduction of the polar substituent on the hydrocarbon substrate have been studied by computing packing energies, their correlation to molecular size, and their apportioning into contributions from each atom in the molecule. The reciprocal orientation of the C=O or C=N dipoles in the crystal has also been analyzed; they never point at each other with the negative charge first. Electrostatic packing energy calculations, in the dipole-dipole approximation, reveal that quite often this energy is negligible and that dipole-dipole interactions are feeble. Centrosymmetric space groups are less frequent in these crystals than in hydrocarbon crystals, and the conditions for formation of head-to-tail pairs between molecular dipoles are discussed. It appears that other cohesive forces can easily overcome this tendency. © 1990 American Chemical Society.