Diprotonated Parabanic Acid: A vicinal or 1,3‐dication?

Reacting parabanic acid with the superacidic systems XF/MF5 (X = H, D; M = As, Sb) in different ratios, led to the formation of the mono‐ and diprotonated species. Salts in terms of [C3H3N2O3][AsF6], [C3H3N2O3][SbF6], [C3H4N2O3][AsF6]2, [C3H4N2O3][SbF6]2, [C3D3N2O3][AsF6] and [C3D4N2O3][AsF6]2 were obtained and characterized by low‐temperature infrared and Raman spectroscopy. Single‐crystal X‐ray structure analyses were performed for [C3H3N2O3][SbF6] and [C3H4N2O3][AsF6]2·4HF. Additionally, quantum chemical calculations were carried out on the B3LYP/aug‐cc‐pVTZ level of theory for the mono‐ and dication. Mapped Electrostatic Potentials together with Natural Population Analysis charges were calculated in order to localize the two positive charges of the diprotonated parabanic acid. The diprotonated parabanic acid can be described as an 1,2‐C,C‐dication, stabilized by electron delocalization over the five‐membered ring.