Sequential Proton Transfer Through Water Bridges in Acid-Base Reactions

The proton transfer mechanism between aqueous Brønsted acids and bases, forming an encounter pair, has been studied in real time with ultrafast infrared spectroscopy. The transient intermediacy of a hydrated proton, formed by ultrafast dissociation from an optically triggered photoacid proton donor ROH, is implicated by the appearance of an infrared absorption marker band before protonation of the base, B – . Thus, proton exchange between an acid and a base in aqueous solution is shown to proceed by a sequential, von Grotthuss–type, proton-hopping mechanism through water bridges. The spectra suggest a hydronium cation H 3 O + structure for the intermediate, stabilized in the Eigen configuration in the ionic complex RO – ···H 3 O + ···B – .