A computational study on molecular structure and stability of tautomers of dipyrrole-based phenanthroline analogue

This work reports structure and stability of different tautomers of a newly-designed analogue of 1,10-phenanthroline-5,6-dione (the di-keto structure (DK) derived from 1H,8H-pyrrolo[3,2-g]indole (PI)) using quantum chemical computations. Molecular structures and relative stabilities of four tautomers and their relevant rotamers of the investigated system have been studied using the B3LYP and M06 methods combined with the 6-31 + G(d,p) basis set, as well as the CBS-QB3 method. The effect of solvent has also been modelled in water using the PCM method at the M06/6-31 + G(d,p) level of theory. Moreover, the aromaticity of the seven structures have been discussed based on their geometrical properties as well as the NICS indices. The results emphasize the predominance of the di-keto isomer (DK) and its greater stability over the keto-enol and di-enol conformers according to structural, energetic and magnetic criteria. In addition, NBO charges, HOMO-LUMO and ESP of the most stable tautomer DK have been employed to predict the electron donation active side toward metal ions.