Capturing an Eigen complex in an acid-base reaction shows step-resolved molecularity

In the Eigen-Weller framework, acid-base reactions are described as those consisting of serial steps. The steps include the encounter of acid and base compounds, short-range proton transfer within the encounter complexes, and separation of the resulting Eigen complexes (ECs) equivalent to long-range proton diffusion. Although the initial proton transfer step in the encounter complexes has been extensively explored, the final step requisite to terminating the acid-base reactions has been overlooked. Using time-resolved fluorescence spectroscopy and chemical kinetics analysis, we track the excited-state proton transfer of a cationic acid to an aprotic base in binary solvent mixtures, where the lifetimes of ECs are prolonged. Identifying the ECs spectrally and kinetically, we investigate the molecularity in the consecutive steps of the hydrogen-bond formation between the acid and base and the dissociation of the EC to unveil the cooperative nature of the aprotic base molecules in the model reaction. • Transient Eigen complex in an acid-base reaction is identified • Molecularity of each elementary step in the acid-base reaction is determined • The aprotic base molecules collaborate to dissociate the proton from the acid The Eigen complex is a key intermediate species in acid-base reactions, yet its identity is rarely explored due to its transient nature. In the present time-resolved spectroscopic study, Lee et al. capture the short-lived Eigen complex spectrally and analyze kinetics to unveil its reactivity in terms of molecularity.