Studies on the π–π stacking features of imidazole units present in a series of 5-amino-1-alkylimidazole-4-carboxamides

Abstract

Reaction of 2-ethoxymethyleneamino-2-cyanoacetamide with primary alkyl amines in acetonitrile solvent affords 1-substituted-5-aminoimidazole-4-carboxamides. Single crystal X-ray diffraction studies of these imidazole compounds show that there are both anti-parallel and syn-parallel π–π stackings between two imidazole units in parallel-displaced (PD) conformations and the distance between two π–π stacked imidazole units depends mainly on the anti/ syn-parallel nature and to some extent on the alkyl group attached to N-1 of imidazole; molecules with anti-parallel PD-stacking arrangements of the imidazole units have got vertical π–π stacking distance short enough to impart stabilization whereas the imidazole unit having syn-parallel stacking arrangement have got much larger π–π stacking distances. DFT studies on a pair of anti-parallel imidazole units of such an AICA lead to curves for ‘π–π stacking stabilization energy vs. π–π stacking distance’ which have got similarity with the ‘Morse potential energy diagram for a diatomic molecule’ and this affords to find out a minimum π–π stacking distance corresponding to the maximum stacking stabilization energy between the pair of imidazole units. On the other hand, a DFT calculation based curve for ‘π–π stacking stabilization energy vs. π–π stacking distance’ of a pair of syn-parallel imidazole units is shown to have an exponential nature.