¹³C NMR spectra of 8-mercaptoquinoline and 8-hydroxyquinoline

Abstract

The signals in the¹³C NMR spectra of quinoline and its 8-substituted derivatives (SH, SCH₃, OH, OCH₃, NH₂, I, and CH₃), 8,8′-diquinolyl disulfide, the 8-hydroxy-N-methylquinolinium ion, and the protonated and anionic forms of 8-hydroxy- and 8-mercaptoquinoline were assigned. The increments of the substituents in the neutral forms of these compounds correlate satisfactorily with those in substituted benzenes and the Swain-Lupton parameters (r = 0.94–0.99). The differences in the ortho increments of the substituents are due to a change in the electron densities on the carbon atoms and also to steric hindrance. The effect of an intramolecular hydrogen bond on the¹³C chemical shift of the quinoline ring of 8-hydroxy- and 8-mercaptoquinoline was examined. The¹³C chemical shifts correlate satisfactorily with the total charges (q) on the carbon atoms in the neutral forms of these compounds. A similar correlation is satisfied to a lesser extent for the protonated and anionic forms because of a change in the bond orders.