engleski

The influence of molecular dipoles on crystal packing of triacylmethanes

Over recent years there has been a growing interest in the use of dipole-dipole interactions in crystal engineering.[1] However the exact limits of the influence of molecular dipole interactions on the solid state organization are far from clear. On the one hand, it is hard not to recognize the importance of local electrostatic interactions in crystal structures of many molecular solids comprising polar molecules. On the other hand, dipolar interactions contribute only slightly to the overall packing energy.[2] Some attention was also given to the connection between molecular dipole magnitude and the centrosymmetry in molecular crystals, with conclusions varying from denying existence of any connection [3] whatsoever, to demonstrating unequivocal preference of molecules with large dipole moments to pack in centrosymmetrical structures.[4] For the subject of our study of dipole interactions in molecular crystals we chose triacylmethanes. Triacylmethanes are acyclical 1, 3, 3’-triketones i. e. triketones with three acyl groups bonded to the same methyne carbon. Their central triketo group is of approximate C3 symmetry rendering the molecule highly polar. 26 such compounds were synthesized and their crystal structures determined. In all these structures dipolar interactions almost entirely determine the molecular aggregation in one dimension. The molecules pack in columns so that their dipole moments are collinear (or quasi-collinear) and aligned in a head to tail manner. Further packing of molecular rows does not appear to be much influenced by molecular dipoles. If molecules can participate in C—H•••O or C—H•••pi hydrogen bonding it often leads to formation of centrosymmetric dimers which are stacked to form double columns in which molecules are placed as described above. A relatively large fraction of triacylmethanes (9 out of 26) was found to crystallize in polar space groups. It does not appear to be any significant correlation between the dipole magnitude and centrosymmetry. There is however a correlation between centrosymmetry and the ability of molecules to form weak hydrogen bonds ; of all triacylmethanes crystallizing in polar space groups only one displays significant C—H•••O contacts. It therefore appears that in triacylmethanes hydrogen bonding has a greater influence on the crystal symmetry than the dipole interactions.

molecular dipole; crystal packing; crystal symmetry

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