Spectroscopic Study of Perchloric Acid Dihydrate H5O2+ClO4−

The infrared and Raman spectra of the H₅O₂⁺ ion have been observed in the crystalline dihydrate of perchloric acid at − 180°C. Existence of H₅O₂⁺ ions in that crystal, first detected by x‐ray diffraction, is further confirmed by the absence of certain characteristic bands of the H₃O⁺ and H₂O species. The spectra are consistent with a centrosymmetric H₂O–H–OH₂ structure of trans configuration approximating the $C_{\text{2h}}$ point group. In the deuterated compound the infrared spectra indicate the same structure for the D₅O₂⁺ ion. Interference from the strong bands of ClO₄⁻ prevented identification of all the fundamentals of H₅O₂⁺.