Hydroxyquinolines: Constitutional isomers and tautomers

The structure, electron population of the ring σ and π valence orbitals, and aromaticity by means of HOMA index were calculated for the 2-, 3-, …, 8-hydroxychinolines (NHQ) at the B3LYP/6-311G∗∗ level. Their conformers formed due to the OH group rotation and the N–H tautomers appeared upon the OH proton shift to the quinoline N-atom were also considered. For all NHQs, but 2HQ, the most stable is an OH tautomer. In the NH-tautomer of the 2HQ molecule, the CO and NH groups are neighboring and constitute cyclic amide: the most stable of all isomers. In comparison to 8HQ, the significant, and mutually opposite, valence orbital population changes appear only when the OH group is attached to the pyrido-ring. In the NH-tautomers, formation of the N–H σ bond withdraws some σ-electrons from the pyrido-ring and releases a similar amount of the π-electrons. Some of the NH tautomers appear to be non-Kekuléan. The aromaticity of the two rings exceeds 0.75 for the OH tautomers, while for the NH tautomers aromaticity of the benzo-ring is from 0.6 (8HQ) to 0.15 (5HQ) for substitution in the benzo-ring and from 0.24 (4HQ) to 0.78 (2HQ) for substitution in the pyrido-ring. The sum of HOMA indices for aromatic OH tautomers exceeds 1.5. Some of the NH tautomers are Kekuléan and some are not. The Kekuléanity/non-Kekuléanity phenomenon was analyzed by means of sum of the HOMA indices, HOMA index of 8-circuit, and sum of Laplacians in the ring critical points.