Vibration-spectral and structural comparison of the 4-aminopyridine cation with the 4-hydroxypyridinium ion and 4-pyridone. The protio parent ions, N- and C-deuterated, and N-methylated ions. Relevant N.M.R. spectral studies

Infrared and Raman spectra have been obtained for the 4-aminopyridine cation and its N?, N?, N?-D₃, 2,3,5,6-D₄, D₇, and 1-Me derivatives, and for the 4-hydroxypyridinium cation and its N, O-D₂, 2,3,5,6-D₄, D₆, and 1-Me derivatives. Throughout, the vibrational spectrum of the 4-aminopyridine cations shows prominent features that differ from those found for the corresponding 4-hydroxy-pyridinium ion, but there is a strong resemblance to the spectrum of the corresponding 4-pyridone. These results are consistent with a structure of the 4-aminopyridine cation represented essentially by the iminium form (II), and inconsistent with one to which the aminopyridinium form (I) makes a large contribution. The downfield shift of the CH proton magnetic resonances on proton addition is much smaller for 4-aminopyridine than for ordinary pyridines, indicating a loss of aromaticity in the cation of the former. The average downfield shift on protonation of 1,4-dihydro-4-imino-1-methylpyridine (V; NNe in place of NH, R = H), too, is somewhat smaller than that found for pyridines, i.e. there is no sign of a gain in aromaticity when the imine (V) forms a cation. All this fully supports the above conclusion.