Abstract
We investigate the acid-base proton exchange reaction in a microsolvated bifunctional chromophore by means of quantum chemical calculations. The UV/vis spectroscopy shows that equilibrium of the keto- and enol-forms in the electronic ground state is shifted to the keto conformation in the excited state. A previously unknown mechanism involving a hydroxide ion transport along a short water wire is characterized energetically, which turns out to be competitive with the commonly assumed proton transport. Both mechanisms are shown to have a concerted character, as opposed to a step-wise mechanism. The alternative mechanism of a hydrogen atom transport is critically examined, and evidence for strong solvent dependence is presented. Specifically, we observe electrostatic destabilization of the corresponding πσ* state by the aqueous solvent. As a consequence, no conical intersections are found along the reaction pathway.
Highlights
Title: Competition between excited state proton and OH− transport via a short water wire: solvent effects open the gate
They will “inject” a proton into the surrounding water, which propagates along the aqueous hydrogen bond network
Our results focus on the competition between the proton injection/recapture process and an alternative pathway featuring initial proton abstraction by the photoacid, followed by effective migration of the OH-cation
Summary
Title: Competition between excited state proton and OH− transport via a short water wire: solvent effects open the gate. The calculations provide evidence for a concerted mechanism of the proton/ hydroxyl transfer along a short water wire.
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