Nature of π-Interactions in Nitrogen-Containing Heterocyclic Systems:  A Structural Database Analysis

Noncovalent interactions involving heteroaromatic ring systems play a major role in determining the function of many chemical as well as biological molecules. Therefore, detailed quantitative analyses of the π-interactions (X−H···π and π···π) were carried out for nitrogen-containing π-systems (isoxazole, imidazole, and indole moieties). Statistical analysis of the geometrical properties for the oxygen, nitrogen, and carbon atom donors with the heterocyclic π-system acceptors showed that carbon donors participate in relatively large numbers for X−H···π interactions, and they adopt T-shaped geometry. The π···π interaction analysis was categorized into three types based on the involvement of the heterocyclic π-systems with themselves, with any other benzene ring present in that particular structure, and their influence on the formation of π-interactions between benzene−benzene rings found in that particular compound. The π-systems in all the three categories prefer to form offset stacking π···π interaction geometry. The benzene rings which normally favor the formation of T-shaped geometry are found to prefer offset stacking geometry, which may be due to the influence of the heteroatom. The π-systems in these heterocyclic structures behave similar to the phenylalanine−phenylalanine interactions in proteins, and therefore this quantitative analysis can serve as a guide for structural and biological studies.