EXPERIMENTAL AND INDO CI CALCULATIONS OF THE ELECTRONIC ABSORPTION-SPECTRA OF ACRIDINE AND 9-ACRIDINAMINE FREE BASES AND THEIR PROTONATED FORMS WITH REGARD TO TAUTOMERIC PHENOMENA

The tautomeric phenomena in 9-acridinamine and its protonated forms were studied employing electronic absorption spectroscopy. The experimental UV spectra revealed that the molecules can most probably exist in amino and imino tautomeric forms. This was confirmed by examining frequencies and oscillator strengths of electronic transitions derived for various structural forms by two independent methods based on the INDO approximation. The latter methods predict that all strong transitions are of the pi --> pi* type and cause a shift in electron density from endocyclic nitrogen to C(9) and exocyclic nitrogen atoms. Comparative studies for acridine and its protonated forms were also carried out to evaluate the soundness of the approach applied.