Real-Time Observation of Bimodal Proton Transfer in Acid-Base Pairs in Water

TLDR

The study aims to refine the Eigen‑Weller picture of acid–base reactions by introducing a three‑stage model that accounts for the observed dynamics. Using femtosecond vibrational spectroscopy, the neutralization of an acid and a base in water after optical excitation was monitored, and the data support the proposed three‑stage model. Bimodal proton‑transfer dynamics were observed: in hydrogen‑bonded complexes the transfer occurs within 150 fs, whereas in encounter pairs the reaction is an order of magnitude slower.

Abstract

The neutralization reaction between an acid and a base in water, triggered after optical excitation, was studied by femtosecond vibrational spectroscopy. Bimodal dynamics were observed. In hydrogen-bonded acid-base complexes, the proton transfer proceeds extremely fast (within 150 femtoseconds). In encounter pairs formed by diffusion of uncomplexed photoacid and base molecules, the reaction upon contact was an order of magnitude slower, in agreement with earlier reported values. These results call for a refinement of the traditional Eigen-Weller picture of acid-base reactions: A three-stage model is introduced to account for all observed dynamics.