Molecular Structure of the Solvated Proton in Isolated Salts. Short, Strong, Low Barrier (SSLB) H-bonds

Abstract

Large, inert, weakly basic carborane anions of the icosahedral type CHB11R5X6 - (R = H, Me; X = Cl, Br) allow ready isolation and structural characterization of discrete salts of the solvated proton, [H(solvent) x ][CHB11R5X6], (solvent = common O-atom donor). These oxonium ion Brønsted acids are convenient reagents for the tuned delivery of protons to organic solvents with a specified number of donor solvent molecules and with acidities leveled to those of the chosen donor solvent. They have greater thermal stability than the popular [H(OEt2)2][BArF] acids based on fluorinated tetraphenylborate counterions because carborane anions can sustain much higher levels of acidity. When organic O-atom donors such as diethyl ether, tetrahydrofuran, benzophenone, and nitrobenzene are involved, the coordination number of the proton (x) in [H(solvent) x ]+ is two. A mixed species involving the [H(H2O)(diethyl ether)]+ ion has also been isolated. These solid-state structures provide expectations for the predominant molecular structures of solvated protons in solution and take into account that water is an inevitable impurity in organic solvents. The O···O distances are all short, lying within the range from 2.35 to 2.48 Å. They are consistent with strong, linear O···H···O hydrogen bonding. Density functional theory calculations indicate that all H(solvent)2 + cations have low barriers to movement of the proton within an interval along the O···H···O trajectory, i.e., they are examples of so-called SSLB H-bonds (short, strong, low-barrier). Unusually broadened IR bands, diagnostic of SSLB H-bonds, are observed in these H(solvent)2 + cations.

Unexpected IR Characteristics of Hydrogen Bonds in the 18-Crown-6-Ether Complex of the H3O+ Hydronium Ion. Can the Location of the Protons Be Specified?

“…The findings have some conceptual relationship to short, strong, low-barrier (SSLB) H bonds in species such as H(O donor) 2 + cations 23,24 where the H atom resides in a nearly flat-bottomed potential well, with a barrier separating two wells that is lower in energy than the first exited vibrational level. Thus, in some complexes H + has an indeterminate position and must be addressed by a more complex theory.…”

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

“…from L is bound directly to H + , the normally strong, sharp, stretching bands associated with these groups broaden, diminish in intensity, or even disappear into the baseline. 17,18,24,[59][60][61] The effect is seen only in symmetrical proton disolvates. For example, both ν as POO and ν s POO bands of crystalline di(methylphenyl)phosphoric or di(chlorophenyl)-phosphoric acid dimers, where the equivalency of the O-atoms in the OHO bridges has been established by X-ray crystallography, disappear in the IR spectra.…”

The Structure of the H3O+ Hydronium Ion in Benzene

“…In O-atom donor solvents, such as ethers, alcohols, and ketones, proton disolvates prevail because of the stability of short, strong, low-barrier (SSLB) H-bonds in linear twocoordinate [solv-H + -solv] + cations. 1,2 Much less is known about the ionization of acids in weak donor solvents, such as arenes or halocarbons, and there is the additional problem of the role of trace water. Its ubiquitous presence in organic solvents means that hydronium ions (H 3 O + , H 5 O 2 + , etc.)…”