Vibrational Spectral Studies of HClO4–DMSO Solutions: Hydrogen Bonding Data for Cation (DMSO)2H+

Abstract

The solutions of anhydrous perchloric acid in dimethyl sulfoxide, denoted as HClO₄·nDMSO (n = 2.0, 3.0, 3.2, 3.7, 8.2, and 16), as well as DClO₄·2DMSO, HClO₄·2DMSO-d₆, and DClO₄·2DMSO-d₆, were first prepared and studied at room temperature using FTIR absorption and Raman spectroscopy in the frequency range 50–4000 cm^–1 with measurements of the depolarization degree for Raman lines. The assignment of observed fundamental frequencies was suggested. It has been shown that the dissolution of HClO₄/DClO₄ in DMSO/DMSO-d₆ results in the complete acid ionization with the formation of ion pairs, such as (DMSO)₂H⁺ClO₄^–, (DMSO)₂D⁺ClO₄^–, (DMSO-d₆)₂H⁺ClO₄^–, and (DMSO-d₆)₂D⁺ClO₄^–. Two solvent molecules inside the complex cation are held together by a proton through the formation of a strong hydrogen bond. The (DMSO)₂H⁺ cation is characterized by a continuous IR band ν_as(OH⁺O) at 1250 cm^–1 and a singlet IR band ν(SO) at 811 ± 10 cm^–1; (DMSO-d₆)₂H⁺ is identified by a continuous band ν_as(OH⁺O) at 1256 cm^–1 and a resonance doublet ν(SO) at 737 and 844 cm^–1; (DMSO)₂D⁺ shows a broad band ν_as(OD⁺O) with a maximum at ~ 877 cm^–1 and a singlet ν(SO) at 800 ± 10 cm^–1; (DMSO-d₆)₂D⁺ is characterized by a broad band ν_as(OD⁺O) at ~ 816 cm^–1 and a resonance doublet ν(SO) at 724 and 870 cm^–1. Physical property studies have shown that solutions with n = 2.0 can crystallize under the influence of small temperature variations. Therefore, their liquid state is metastable.