The 1H NMR spectra of quinoline, quinoline N-oxide, the quinolinium ion and their monomethyl derivatives

1H chemical shifts of quinoline, quinoline N-oxide and the quinolinium ion were obtained by complete analysis of their NMR spectra and interpreted critically in an attempt to quantify the possible different effects acting on the shielding constant of protons in these systems. Semi-empirical SCF calculations of electron charge densities and ring current contributions were also performed. It was found that the same effects that act on the shielding of protons directly bonded to the heterocyclic system also act, in an attenuated form, on the proton chemical shift of methyl groups in monomethyl derivatives. Vicinal coupling constants were rationalised in terms of -electron bond order and electro-negativity of neighbouring atoms.