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 C2h 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+.