Spectrophotometric investigations of protonated forms of heterocyclic compounds

2,4,6-trimethylpyridine, quinoline, 2-methylquinoline, 4-chlorine-2-methylquinoline, and acridine aromatic heterocycles, as well as protonated forms thereof, are studied by electronic absorption spectroscopy in solution. The influence of protonation on the electronic absorption spectra of heterocycles consists in a long-wavelength shift of the absorption spectrum in comparison with the absorption of initial molecules. The spectra of the protonated forms of all compounds exhibit a relative increase in the intensity of the long-wavelength absorption band compared to the short-wavelength band and a smearing of the vibrational structure in the long-wavelength band.