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

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.