engleski

π – π Interactions in Picrates of Aminobenzoic Acids – True or False?

Although interactions between systems of π electrons are generally repulsive, and stacking of aromatic rings is better attributed to other causes, attractive π∙∙∙π may occur if a π→π* charge transfer occurs between neighbouring molecules. Such a charge transfer is characterised by the occurrence of charge-transfer bands in the UV-Vis spectrum, and is visible through a change in the colour. Such a charge transfer is more likely when molecules have strongly electron withdrawing or donating properties or are themselves already charged. Good models for the latter case are picrates of aromatic amines. Two series of aminobenzoic acid picrates salts, those of 1:1 and those of 2:1 stoechiometry, as well as cocrystals with urea, have been prepared and studied. Although the solutions of all these compounds are bright yellow (characteristic for picrate anion), the colours of the crystals vary from yellow to brown or red. The spectra of non-yellow solids show appearance of wide bands associated with charge transfer. An analysis of the structures was performed to determine whether the observed charge transfer can be connected with some particular features of the stacking of the aromatic rings and other structural motifs. The stacking is usually such that there is a minimal overlap of π-systems. On the other hand, the hydrogen atoms participating in hydrogen bonding appear in many cases to be “smeared” along the hydrogen bond This would indicate that the charge transfer in picrates of aminobenzioc acids is more likely mediated by the migration of hydrogen atoms along the hydrogen bonds, than by π→π* electron transitions.

picric acid; picrates; pi-pi interactions; aminobenzoic acids

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