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Abstract

Salts of the C_3v 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 C_3v 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 C_3v 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 C_3v 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 ν_max(OH) and δ(OH₃) within the ranges 3010−2536 cm^-1 for ν_max(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 C_3v symmetry are suggested.

Citing Articles

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This article has been cited by 14 ACS Journal articles (5 most recent appear below).

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  Isobutene Polymerization Using Chelating Diboranes: Polymerization in Aqueous Suspension and Hydrocarbon Solution

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