Stepwise formation of proton hydrates H+·NH2O·pTBP·Cl- in tributyl phosphate solutions of HCl: composition, structure and peculiarities of IR spectra

Abstract

Stepwise formation of several H⁺·NH₂O·pTBP proton hydrates in tributyl phosphate (TBP) solutions of HCl has been studied by IR spectroscopy. As the water content in solutions increases, sequential formation of four compounds (H₃O⁺–Cl^-·TBP)_n (I), (H₅O₂⁺·Cl^-·TBP)_m (II), (H₅O₂⁺·Cl^-·H₂O·TBP)_p (III) and [H₅O₂⁺·Cl^-·2H₂O·TBP] (IV) has been found. A generic connection between the structures of I–IV and particular features of their IR spectra has been established. The proton monohydrate in I should not be considered as a hydroxonium ion H₃O⁺, but rather as a molecular fragment of a proton disolvate of the B₁–H⁺–B₂ type, where B₁=H₂O and B₂=Cl^-. As a stronger proton acceptor in comparison with FeCl₄^- and ClO₄^- ions, the Cl^- anion has a direct contact with the H₅O₂⁺ cation in II–IV. As a result, only H₂O molecules of the first co-ordination sphere of the H₅O₂⁺ distributed cation are spectroscopically distinguishable from more distant H₂O molecules of the surroundings. It has been shown that the main features of the IR spectra of the H₅O₂⁺ cation, including the so-called continuous background absorption (CBA), are due to its intrinsic properties and to the influence of molecules of the first co-ordination sphere that are bound to it. Water molecules of the second and following hydration spheres of the contact ion pair H₅O₂⁺·Cl^- do not feel the proton charge and do not affect CBA to a noticeable extent.