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 [C₃ H₅ O₃ ]⁺ [SbF₆ ]^- , [C₃ H₅ O₃ ]⁺ [AsF₆ ]^- and [C₃ H₄ DO₃ ]⁺ [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 [C₃ H₅ O₃ ]⁺ [Sb₂ F₁₁ ]^- , 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 [C₃ H₅ O₃ ]⁺ 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 ¹³ C NMR-spectroscopic study of [C₃ H₅ O₃ ]⁺ [SbF₆ ]^- .