Infrared spectroscopic estimation of H+ and D+ positions in the sructure of the orthorhombic crystals of H5O 2 + ClO 4 − and D5O 2 + ClO 4 −

The crystalline samples of the orthorhombic phase of H5O2+ClO4− (I) and D5O2+ClO4− (II) are examined by infrared (IR) absorption spectroscopy. Inaccuracies referring to the identification of II found in the literature are corrected. The work shows that a typical evidence of the crystallization of I is the doublet splitting ν(OH) at 3232 cm−1 and 3340 cm−1; and a typical evidence of the crystallization of II is a similar doublet splitting ν(OD) at 2391 cm−1 and 2484 cm−1. In order to determine the doublet origin and to make clear how it depends on the concentration of hydrogen isotopes, specially prepared isotopically mixed crystalline samples of the composition (H1−xDx)5O2+ClO4− (III, x = 0.17–0.90) are studied. Observations of the concentration dependence of the doublet splitting are used to check the quality of the existing variants of the crystal structure of I. Agreement in the experiments with the samples of III is obtained when the structural model of the cation with the H+ and D+ positions in the center of a strong H2O…H+…OH2 and D2O…D+…OD2 hydrogen bond (HB) is used. Although we do not exclude the possibility of a symmetrical statistical distribution of H+ and D+ between two potential energy minima. The results of the spectroscopic analysis agree with the known data of the X-ray crystallographic analysis, showing that the crystals of I have a space symmetry group Pnma. At the same time, they disagree with the recently published data of theoretical calculations of the crystal structure of I (P212121), these data showing that H+ notably shifts from the center of the strong HB towards an oxygen atom of one of the two water molecules.