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 −

Abstract

The crystalline samples of the orthorhombic phase of 5 2 4 Н O ClO + − (I) and 5 2 4 D O ClO + − (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(ОН) at 3232 cm -1 and 3340 cm -1 ; and a typical evidence of the crystallization of II is a similar doublet splitting ν(ОD) 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 1 5 2 4 (H D ) O ClO x x + − − (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 Н 2 О…Н + …ОН 2 and D 2 О…D + …ОD 2 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 (P2 1 2 1 2 1 ), 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. INTRODUCTIONThe peak of interest in the structural analysis of crystalline hydrates of perchloric acid and some other strong acids was in the second half of the last century. At that period, the structure of some crystalline hydrates of perchloric acid, including the 5 2 4 H O ClO + − crystals with a melting point of 20.65 °C [1], was determined by the X-ray crystallography. At the final stage of the structure identification, the symmetry space groups Pnma and Pn2 1 a were found to be equally possible: the R-factor of 0.114 and 0.108 respectively. A preference was given to the centrosymmetric group, though, in particular, its Rfactor was higher. If there were the space group Pnma, then the H + proton would be in the center of crystallographic inversion, and, consequently, in the center of a strong Н 2 О…Н + …ОН 2 hydrogen bond. However, in the final chapter of [1] 222 and in the chapter, devoted to the X-ray crystallography data of [2], there was a note regarding the known limitations of the method used. This note indicates that the X-ray diffraction method does not allow the determination of the H + location: whether it is on the symmetry element or its opposite sides, occupying two statistically or dynamically related positions.Using the single crystal X-ray diffraction (XRD) data of [1], the IR and Raman spectra of 5 2 4 H...

Metastable crystalline phases of D5O 2 + ClO 4 − : Selective stabilization by addition of the light hydrogen isotope

“…We hesitate to state that the list of actual metastable crystalline phases of of НClO 4 ⋅ 2Н 2 O was cooled at a rate of up to 60 deg/min. However, in [5], where such experiments were performed, and in the present work, no metastable phase was found, except for the well-known orthorhombic phase, whose structure is similar to that of a. To explain this, we analyzed the composition of the solid phases formed during crystallization of partially deuterated vitreous samples of perchloric acid dihydrate.…”

“…Information on its existence is given in [4,5]. This phase was also detected in the monotropic transformation of phase b.…”

“…The metastable character of phase a isostructural with 5 2 4 Н O ClO + − is far from appearing in each experiment, which is established in [4,5]. Being relatively thermally stable, it can easily be melted without polymorphic transformation.…”

“…Solutions of DClO 4 ⋅2D 2 O with 99 at.% D, prepared by a standard method [4,5], as well as their liquid mixtures with НClO 4 ⋅2Н 2 O of different composition were crystallized in a nitrogen vacuum IR cryostat with external KBr windows.…”

“…The criterion for the phase transformation was frequency shifts, the disappearance of the original bands, and the appearance of new bands. Further details regarding the experimental technique can be found in [4,5].…”