603. The vibration spectra and structures of the hydrochlorides of aminopyridines

Abstract

The infrared and Raman spectra of the hydrochlorides of 2-, 3-, and 4-aminopyridine, of 2-aminopyrimidine, and of the 3-, 4-, 5-, and 6-methyl derivatives of 2-aminopyridine, have been compared with those of the free amines. Whereas the vibration spectrum of 3-aminopyridine hardly changes on cation formation those of the α- and γ-amino-compounds change considerably, showing that there is a further structural change beyond mere proton addition. The spectral change being too drastic to be explained simply by cation structures in which the forms (IV) and (V), and (VIII) and (IX), contribute about equally, it is concluded that the cations are of the amidinium type and are represented essentially or largely by structures (V) and (IX). In agreement with this idea, the vibration spectrum of the 4-aminopyridine cation strongly resembles that of 4-pyridone (X) and differs considerably from that of the 4-methoxypyridine cation.