The Nature of the H3O+ Hydronium Ion in Benzene and Chlorinated Hydrocarbon Solvents. Conditions of Existence and Reinterpretation of Infrared Data

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Abstract
Salts of the C3v symmetric hydronium ion, H3O+, have been obtained in the weakly basic solvents benzene, dichloromethane, and 1,2-dichloroethane. This is made possible by using carborane counterions of the type CHB11R5X6- (R = H, Me, Cl; X = Cl, Br, I) because they combine the three required properties of a suitable counterion: very low basicity, low polarizability, and high chemical stability. The existence of the H3O+ ion requires the formation of three more-or-less equivalent, medium-to-strong H-bonds with solvent or anion bases. With the least basic anions such as CHB11Cl11-, IR spectroscopy indicates that C3v symmetric trisolvates of formulation [H3O+ ·3Solv] are formed with chlorocarbon solvents and benzene, the latter with the formation of π bonds. When the solvents and anions have comparable basicity, contact ion pairs of the type [H3O+·nSolv·Carborane] are formed and close to C3v symmetry is retained. The conditions for the existence of the H3O+ ion are much more exacting than previously appreciated. Outside of the range of solvent basicity bounded at the lower end by dichloromethane and the upper end by tributyl phosphate, and with anions that do not meet the stringent requirements of weak basicity, low polarizability of high chemical stability, lower symmetry species are formed. One H-bond from H3O+ to the surrounding bases becomes stronger than the other two. The distortion from C3v symmetry is minor for bases weaker than dichloromethane. For bases stronger than tributyl phosphate, H2O−H+−B type species are formed that are more closely related to the H5O2+ ion than to H3O+. IR data allow criteria to be defined for the existence of the symmetric H3O+ ion. This includes a linear dependence between the frequencies of νmax(OH) and δ(OH3) within the ranges 3010−2536 cm-1 for νmax(OH) and 1597−1710 cm-1 for δ(OH3). This provides a simple way to assess the correctness of the formulation of the proton state in monohydrated acids. In particular, the 30-year-old citation classic of the IR spectrum believed to arise from H3O+ SbCl6- is re-interpreted in terms of (H2O)x·HSbCl6 hydrates. The correctness of the hydronium ion formulation in crystalline H3O+A- salts (A- = Cl-, NO3-) is confirmed, although, when A- is a fluoroanion, distortions from C3v symmetry are suggested.

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