The relative signs of NMR proton–carbon coupling constants in quinolines

The relative signs of spin–spin coupling constants are of practical significance in spectral analysis and in structural elucidation. The signs, however, cannot be obtained directly from usual NMR spectra. In the present study, we measured 1H and 13C NMR spectra of three quinolines and used the Selective Population Transfer (SPT) to determine the relative sign of their proton–carbon coupling constants. For the determination of the relative signs of JCH, hetero-nuclear SPT experiments accompanied with proton decoupling were applied to systems in which a carbon nucleus couples with four or more protons. SPT was applied to the lowest or highest field 13C satellite of the 1H spectrum. The value of 2JH3C2 of 6-chloro-2-methyl-quinolines (1) was determined to be +2.2 Hz by hetero-nuclear SPT with the decoupling of CH3 protons. Similarly, 2JH6C7 of 7-chloro-8-methylqionoline (2) and 2JH7C8 of 8-hydroxyquinoline (3) were determined to be −2 and −4.0 Hz with the decoupling of CH3 protons of Compound (2) and H4 of Compound (3), respectively.

► The relative signs cannot be obtained directly from usual NMR spectra.
► We used the SPT spectra to determine the relative sign of their proton-carbon coupling constants.
► The SPT enabled us to reduce the 3 or 4 system spin systems of the compounds by decoupling a proton.
► From the arrangement of their energy levels, we deduced the relative sign of the coupling constants.
► The signs of 2JHC or 4JHC of aromatic carbon substituted by an electronegative heteroatom are negative.