Reactions of nitronium sulfates: Hunting for dinitro sulfate

Nearly 70 years ago, it was shown that the reaction between equimolar amounts of SO₃ and N₂O₅ results in dinitronium disulfate (NO₂)₂S₂O₇ rather than monosulfate (NO₂)₂SO₄. Because SO₃ can dimerize, it was believed for long that this feature is the main cause of the formation of nitronium disulfate. Then, it was discovered that the reaction of NOSO₃Cl with N₂O₅ results in dinitronium disulfate, and (NO₂)₂SO₄ was suggested as an intermediate. The development of Raman spectroscopy made it possible for us to solve the fundamental problem of the existence of an elusive dinitro sulfate. The inevitability of dinitro sulfate formation was demonstrated by the transformations of nitronium sulfates (NO₂)₂S₂O₇, NO₂HSO₄, and (NO₂)[CF₃C(O)OSO₃], and the Raman spectroscopy technique has shown the presence of the compound in the solid reaction products. In order to establish the structure of solid dinitro sulfate, quantum chemical calculations of the nitro ester form (O₂NOSO₃NO₂) and of the [OSO₃NO₂]⁻ anion were performed, but the conclusions about the diionic structure [NO₂]⁺₂[SO₄]²⁻ were made only due to the synthesis of the related nitronium sulfates K (NO₂)SO₄ and K (NO₂)S₂O₇ whose Raman spectra were compared with that of dinitro sulfate. The decomposition pathways of all these unstable molecules were established. Reactions of nitronium disulfates, (NO₂)₂S₂O₇ or (NO₂)[CF₃C(O)OS₂O₆], with CF₃COOH showed that, unlike HNO₃, the organic acid does not destroy the sulfate chains but can substitute terminal groups including nitronium ion. So the chemistry of nitronium monosulfates was studied for the first time, and the existence of an unstable dinitronium sulfate was proved using Raman spectroscopy.