On the Origin of the Color in the Solid State. Crystal Structure and Optical and Magnetic Properties of 4-Cyanopyridinium Hydrogensquarate Monohydrate

The novel hydrogensquarate salt of 4-cyanopyridine was synthesized, and its structure and properties were elucidated in detail spectroscopically, thermally, and structurally, using single-crystal X-ray diffraction, linear-polarized solid-state IR spectroscopy, UV spectroscopy, TGA, DSC, DTA, and positive and negative ESI MS as well as 1H and 13C NMR methods. Quantum chemical methods were used to calculate the electronic structure, vibrational data, and electronic spectra. 4-Cyanopyridinium hydrogensquarate monohydrate crystallizes in the space group P1̄ and exhibits a layered structure with molecules linked by intermolecular NH···O(HSq −) (2.651 Å) and HOH···O(HSq −) (2.792 and 2.563 Å) hydrogen bonds with participation of cations, anions, and the solvent molecule. The formation of stable layers of the type (2HSq- ·2H2O) n and the observation of a red color in the solid state is discussed. The optical and magnetic properties were elucidated in comparison to the data for neutral 4-cyanopyridine as well as its four known salts.