The unique nature of H+ in water

The H+(aq) ion in ionized strong aqueous acids is an unexpectedly unique H13O6+ entity, unlike those in gas phase H+(H2O)n clusters or typical crystalline acid hydrates. IR spectroscopy indicates that the core structure has neither C3v H3O+ Eigen-like nor typical H5O2+ Zundel-like character. Rather, extensive delocalization of the positive charge leads to a H13O6+ ion having an unexpectedly long central O O separation of ~2.57 A ° and four conjugated O O separations of ~2.7 Å. These dimensions are in conflict with the shorter O/O separations found in structures calculated by theory. Ultrafast dynamic properties of the five H atoms involved in these H-bonds lead to a substantial collapse of normal IR vibrations and the appearance of a continuous broad absorption (cba) across the entire IR spectrum. This cba is distinguishable from the broad IR bands associated with typical low-barrier H-bonds. The solvation shell outside of the H13O6+ ion defines the boundary of positive charge delocalization. At low acid concentrations, the H13O6+ ion is a constituent part of an ion pair that has contact with the first hydration shell of the conjugate base anion. At higher concentrations, or with weaker acids, one or two H2O molecules of H13O6+ cation are shared with the hydration shell of the anion. Even the strongest acids show evidence of ion pairing. © The Royal Society of Chemistry 2011.