A distinctive feature in the IR spectra of proton disolvates [L2H+] and polysolvates [(L2H+)·nL]: unusual strong broadening of some absorption bands of ligands L bound with H+

Abstract

An IR study of proton disolvates in organic solutions of L₂H⁺FeCl₄⁻, where L=tributyl phosphate (TBP), butyl acetate, diethyl ether, acetone and ethanol, or proton polysolvates of L₂H⁺·nL·FeCl₄⁻ or (L′H⁺L)L′·FeCl₄⁻ (L=ethanol, L′=TBP) under the weak polarizing influence of FeCl₄⁻ anion on the cation led to the discovery of an unusual, very strong broadening of the absorption bands of the molecular groups of ligands L, which are directly bound with H⁺. These bands belong to (i) highly characteristic vibrations of double bonds PO(–H⁺), CO(–H⁺), etc., weakly mixed with the vibration of conjugated bonds, and (ii) strongly interacting group vibrations experiencing the influence of the proton. This effect is best exhibited in the IR spectra of proton solvates H⁺·nL with the positive charge located mainly on one O–H⁺–O group. The following explanation of this effect has been proposed. The potential function of the proton in the O–H⁺–O group is almost a single minimum with a flat bottom. Therefore, H⁺ is delocalized in a certain spatial interval between the oxygen atoms. As a result, the force constants of the bands of ligands L influenced by H⁺ change with the same frequencies as the proton migrates at the bottom of the flat potential minimum. This frequency may be comparable to those of some normal vibrations or exceed them. It has been shown that the same effect also takes place in the IR spectra of proton hydrates H₅O₂⁺· nL, where L=H₂O or organic bases. The effect reported can form a basis for a new interpretation of the peculiarities in the IR spectra of proton hydrates.