Spectroscopic and structural elucidation of 4-dimethylaminopyridine and its hydrogensquarate.

A distortion of the aromatic character and stabilization of the imino-form as a result of the protonation of 4-dimethylaminopyridine was established by IR-, UV- and 1H NMR-spectral analysis of 4-diaminopyridinium hydrogensquarate. Quantum chemical calculations were carried out at MP2 and B3LYP levels of theory and a 6-311++G** basis set with a view to determining the changes in geometrical parameters and IR-spectroscopic characteristics resulting from Npy protonation. Linear-dichroic IR-spectroscopy coupled with the orientation techniques of solid samples as liquid crystal suspensions and melted solid polycrystalline films was applied for the identification of the IR-bands, characteristic for the structural fragments of the neutral and imino-form of the pyridine derivative. The spectral results were compared with the structure, obtained by a single crystal X-ray analysis. The salt contains dimmers of hydrogensquarate anions and Npy protonated cations of which the former are stabilized by strong intermolecular OH...O interactions (2.552 angstroms and 143.1(2) degrees). The 4-diaminopyridinium cation interacts with the anion through moderate NH...O bonds (2.729 angstroms and 165.0(0) degrees ). Individual cations are pi-pi stacked with their neighbors at a distance of 3.406 angstroms.