Myths about the Proton. The Nature of H⁺ in Condensed Media

Abstract

CONSPECTUS Recent research has taught us that most protonated species are decidedly not well represented by a simple proton addition. What is the actual nature of the hydrogen ion (the “proton”) when H+, HA, H2A+ etc. are written in formulae, chemical equations and acid catalyzed reactions? In condensed media, H+ must be solvated and is nearly always di-coordinate – as illustrated by isolable bisdiethyletherate salts having H(OEt2)2+ cations and weakly coordinating anions. Even carbocations such as protonated alkenes have significant C-H---anion hydrogen bonding that gives the active protons two-coordinate character. Hydrogen bonding is everywhere, particularly when acids are involved. In contrast to the normal, asymmetric O-H---O hydrogen bonding found in water, ice and proteins, short, strong, low-barrier (SSLB) H-bonding commonly appears when strong acids are present. Unusually low frequency IR νOHO bands are a good indicator of SSLB H-bonds and curiously, bands associated with group vibrations near H+ in low-barrier H-bonding often disappear from the IR spectrum. Writing H3O+ (the Eigen ion), as often appears in textbooks, might seem more realistic than H+ for an ionized acid in water. However, this, too, is an unrealistic description of H(aq)+. The dihydrated H+ in the H5O2+ cation (the Zundel ion) gets somewhat closer but still fails to rationalize all the experimental and computational data on H(aq)+. Researchers do not understand the broad swath of IR absorption from H(aq)+, known as the “continuous broad absorption” (cba). Theory has not reproduced the cba, but it appears to be the signature of delocalized protons whose motion is faster than the IR timescale. What does this mean for reaction mechanisms involving H(aq)+? For the past decade, the carborane acid H(CHB11Cl11) has been the strongest known Brøsted acid. (It is now surpassed by the fluorinated analogue H(CHB11F11).) Carborane acids are strong enough to protonate alkanes at room temperature, giving H2 and carbocations. They protonate chloroalkanes to give dialkylchloronium ions, which decay to carbocations. By partially protonating an oxonium cation, they get as close to the fabled H4O2+ ion as can be achieved outside of a computer.

Carboranes in the chemist's toolbox

“…5,6 By this means, isolable C 6 H 7 + , n-C 6 H 13 + , and CMe 3 + salts of HCB 11 F 11 − have been generated from benzene, n-hexane, and (remarkably) n-butane, respectively. Chemistry graduate of Lafayette College and received his Ph.D. training with William N. Lipscomb at the carbons at room temperature without oxidation.…”

Effect of quantum nuclear motion on hydrogen bonding

“…In a recent review about the solvation of protons, Reed noted the importance of this parameter regime: "In contrast to the typical asymmetric H-bond found in proteins (NH· · · O) or ice (OH· · · O), the short, strong, low-barrier (SSLB) H-bonds found in proton disolvates, such as H(OEt 2 ) + 2 and H 5 O + 2 , deserve much wider recognition" [16].…”

Solutions of complex copper salts in low-transition-temperature mixture (LTTM)

“…7-10 is the localization of the hydroxyl group proton split out due to complex formation. 26 Crystal structures of more complex aggregates of ammonium cations and ammonia molecules with short and strong hydrogen bonds are also known. This concept mainly considers migration of protons via chains of water molecules with formation of oxonium ions, although special attention is also paid to ammonium cations and hydrogen bonds formed between them and N-heterocycles or amino groups.…”