A study by ultraviolet spectroscopy on self-association of purine, 6-methylpurine, benzimidazole, and imidazo [1,2- a]pyridine in aqueous solution

Abstract

The self-association of four heteroaromatic compounds with fused five- and six-membered rings: purine, 6-methylpurine, benzimidazole, and imidazo[1,2- a]pyridine, was studied in aqueous solution at different pH values, by ultraviolet spectroscopy. The variation in molar absorptivity with the concentration of these compounds was measured for the band systems in the mid-ultraviolet region. Hypochromic deviations from the Beer–Lambert law with increasing concentration were found for all compounds. These results were interpreted in terms of self-association processes involving the formation of dimers and, in certain cases, also the formation of polymers. From the fitting of the experimental curves of hypochromic effects, self-association constants for dimerization and polymerization were calculated. The results obtained are discussed in relation to the self-association properties of the building blocks of each compound, studied by us in previous works. The relevance of the position of the nitrogen atoms at the five- and six-membered rings was analyzed to elucidate the nature of the self-association processes, in particular the tendency to polymerization. The values of the calculated self-association constants are discussed in relation to the intermolecular interaction energies of these compounds in aqueous solution.