Ground and lowest excited triplet state dissociation constants of the 5-, 6- and 7-mononitro-8-quinolinols. Spectroscopic evidence for intramolecular hydrogen bonding in the lowest triplet state of 7-nitro-8-quinolinol.

Abstract

Abstract— The dissociation constants of 5‐nitro‐8‐quinoIinol, 6‐nitro‐8‐quinolinol and 7‐nitro‐8‐quinolinol have been determined for the ground states of the molecules by potentio‐metric titration, and for the lowest triplet states from the shifts of the O—O bands of phosphorescence occurring upon dissociation. The ground‐state dissociation constants of the phenolic groups are found to be typical of ortho, meta, and para nitro phenols. The phosphorescences of all prototropic species derived from the 6‐isomer appear to be anomalous and their origin is unknown. The anomalously low acidity of the neutral species of the 7‐isomer in its triplet state, relative to that of the neutral species of the 5‐isomer in its triplet state, is attributed to intramolecular hydrogen bonding in the lowest triplet sfate of the 7‐isorner, between the phenolic proton and the 7‐nitro group. The magnitudes of the two triplet‐state disSociation constants of both the 5 and 7 isomers suggest that the predominant uncharged species in fluid solutions containing the triplet‐state molecules are the excited zwitterions.