STUDIES ON AMINOPYRIDINES IN AQUEOUS-SOLUTION BY LASER RAMAN-SPECTROSCOPY

Raman spectra of 2-, 3-, 4-aminopyridine and 3,4-diaminopyridine in water solution are reported for the first time in the 4000-300 cm-1 range. Vibrational assignments have been made for many of the observed frequencies on the basis of isotopic frequency shifts, depolarization ratios, group frequency considerations as well as comparison with accepted assignments for certain vibrational modes in other compounds with structural similarities. The Raman spectra of these monoaminopyridines can be interpreted as if the molecule of 4-aminopyridine possesses C2v symmetry and as if the 2- and 3-aminopyridine molecular species were planar, due to a low barrier to amino group inversion. The Raman spectra of 3,4-diaminopyridine, however, preclude any symmetry. This is probably due to the formation of intramolecular hydrogen bonds between the amino groups.