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.