Polarized i.r. spectra of 4-aminopyridine hemiperchlorate single crystal

Abstract

The polarized i.r. spectra of 4-aminopyridine hemiperchlorate single crystals containing structurally asymmetric NHN + bridges were studied at room and liquid nitrogen temperatures. It was shown that the broad intense absorption over the frequency range 200–3200 cm −1 should be assigned to the protonic ν(NHN) stretching vibrations. The transmission windows can be ascribed most probably to a coupling of ν(NHN) vibrations with deformation δ(NHN) and γ(NHN) modes and their overtones as well as with internal vibrations of the pyridine ring which lead to a modulation of the bridge geometry. The behaviour of NH 2 groups in polarized i.r. spectra confirms the inequivalence of 4-aminopyridine moieties in homoconjugated cations both from the point of view of intramolecular charge distribution and environment of NH 2 groups in the lattice, in agreement with the diffraction data.