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

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 CHB 11R5X6-(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 H 3O + 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 H 3O + 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 H 3O + SbCl6is reinterpreted in terms of (H2O)x‚HSbCl6 hydrates. The correctness of the hydronium ion formulation in crystalline H3O + Asalts (A-) Cl-, NO3-) is confirmed, although, when Ais a fluoroanion, distortions from C3v symmetry are suggested.

IR Spectrum of the H5O2+ Cation in the Context of Proton Disolvates L−H+−L

“…This order is similar but not identical to that obtained with H 3 O + salts: CH 2 Cl 2 < {Cl 11 } -solid < {Me 5 Br 6 } -solid ≈ benzene < DCE. 9 The somewhat different ordering may arise from mixing solid-state data with solution data and from the differing sensitivities of the H 5 O 2 + and H 3 O + ions to the polarizability of the base. The H 5 O 2 + ion forms much weaker H-bonds than H 3 O + and is therefore less able to polarize a base.…”

“…9 Conditions for its existence in condensed media are surprisingly exacting. Only solvents in the basicity range of chlorocarbons to trialkyl phosphates can form the three medium-to-strong, nearequivalent, H-bonds necessary to stabilize the trisolvated [H 3 O + ‚ 3L] ion (L ) solvent and/or anion).…”

“…For the lowest basicity solvents, it is necessary to use anions that are not only weakly basic but that have low polarizability and high chemical stability. Outside of these conditions, the H 3 O + ion easily distorts away from C 3V symmetry, forming unsymmetrical disolvates of type H 2 O-H + -L. 9 The H 5 O 2 + ion is prototypical of a large class of symmetrical disolvates, L-H + -L, and the ease of disolvate formation begs the question of under what conditions so-called Zundel-type 10 Proton disolvates such as the H 5 O 2 + ion are stabilized by short, strong, low-barrier (SSLB) H-bonds. 10,12,13 The gas-phase bond dissociation enthalpy of H 5 O 2 + into H 3 O + and H 2 O is ca.…”

Unprecedented Sulfone‐Functionalized Metal–Organic Frameworks and Gas‐Sorption Properties

“…It is important to note here that the C 3 axis found on the atom O15 is in agreement with the expected C 3v symmetry of hydronium H 3 O + cations. [12] These results explain why the synthesis of UoC-1 strictly requires the use of a DMF/H 2 O mixture as solvent instead of DMF only.…”

The Structure of (H3O)2B12F12·6H2O – a CCP Lattice of B12F122– Anions Intercalated with a Nonplanar Network of O–H···O Connected O6 Rings

“…at the air/water interface, where H 3 O + ions may be concentrated relative to the bulk solution), [22,23] and (iii) as free H 3 O + ions [24] or as [(…”