Tautomeric Equilibria in Solutions of 1-Phenacylpyridazin-1-ium and 1-(o-Fluorophenacyl)pyridin-1-ium Cations. DFT and NMR Studies

Although 1-(2-oxo-2-phenylethyl)pyridazin-1-ium and 1-[2-oxo-2-(o- fluoro phenyl - ethyl)]pyridin-1-ium cations may equilibriate with the respective enol and enaminone tautomeric forms, both experimental NMR data and results of ab in itio calculations show the keto forms to be the only species present in DMSO solution. Intramolecular hydrogen bonds and pi electron delocalization seem to have negligible effect on stabilization of the tautomers. On the other hand, electrostatic repulsions between the onium nitrogen and hydroxy oxygen atoms in the compounds studied are considered to be responsible for the in stability of the enol and enaminone tautomeric forms.