Protonated Ethylene Carbonate: A Highly Resonance-Stabilized Cation

Salts containing the monoprotonated ethylene carbonate species of were obtained by reacting it with the superacidic systems XF/MF₅ (X=H, D; M=Sb, As). The salts in terms of C₃H₅O₃]⁺SbF₆]^?, C₃H₅O₃]⁺AsF₆]^? and C₃H₄DO₃]⁺AsF₆]^? were characterized by low-temperature infrared and Raman spectroscopy. In order to generate the diprotonated species of ethylene carbonate, an excess of Lewis acid was used. However, this only led to the formation of C₃H₅O₃]⁺Sb₂F₁₁]^?, which was characterized by a single-crystal X-ray structure analysis. Quantum chemical calculations on the B3LYP/aug-cc-PVTZ level of theory were carried out for the C₃H₅O₃]⁺ cation and the results were compared with the experimental data. A Natural Bond Orbital (NBO) analysis revealed sp² hybridization of each atom belonging to the CO₃ moiety, thus containing a remarkably delocalized 6π-electron system. The delocalization is confirmed by a ¹³C NMR-spectroscopic study of C₃H₅O₃]⁺SbF₆]^?.