Intermolecular interaction studies in ammonium squarate: crystal structure and vibrational spectra

Ammonium squarate salt [(NH4)2C4O4] crystallizes in the monoclinic space group P21/c. The crystal presents the squarate ions displayed in layers parallel to a crystallographic axis and forming hydrogen bonds interactions with NH4+ cations, localized between squarate anions layers. The squarate anion presents similar CC and CO bond distances which indicate a degree of electronic delocalization in the rings. A π-stacking interaction is observed between squarate rings separated by interplanar distance and centroid ring distance of respectively 3.41 and 3.71 Å. The electron delocalization was also observed in the vibrational spectra. The infrared spectrum shows a narrow band around 1530 cm−1, assigned to a coupled stretching mode of CO and CC groups, indicating the high symmetry of the squarate ion. The Raman spectrum also shows this effect in the 1000 to 1200 cm−1 region (related to CC stretching mode), where it is expected a decrease of the number of bands if compared to squarate ion in aqueous solution, where the symmetry is actually D4h; in the (NH4)2C4O4 Raman spectrum just one strong band is observed at 1120 cm−1. All the vibrational results indicate the symmetry is very close to the one observed for the solvated ion in aqueous solution; once again the vibrational analysis appears as an important tool in recognizing the oxocarbon ion geometry in the solid state.