Stepwise formation of proton hydrates H+center dot NH2O center dot pTBP center dot Cl- in tributyl phosphate solutions of HCl: composition, structureand peculiarities of IR spectra

Stepwise formation of several H+ . NH2O . pTBP proton hydrates in tributyl phosphate (TBP) solutions of HCl has been studied by IR spectroscopy. As th e water content in solutions increases, sequential formation of four compou nds (H3O+-Cl- . TBP)(n) (I), (H5O2+ . Cl- . TBP)(m) (II), (H5O2+ . Cl- . H2 O . TBP)(p) (III) and [H5O2+ . Cl- . 2H(2)O . TBP] (IV) has been found. A g eneric 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 frag ment of a proton disolvate of the B-1-H+-B-2 type, where B-1 = H2O and B-2 = Cl-. As a stronger proton acceptor in comparison with FeCl4- and ClO4- io ns, 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 H5O2distributed 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 c ontact ion pair H5O2+ . Cl- do not feel the proton charge and do not affect CBA to a noticeable extent.