Competition between excited state proton and OH⁻ transport via a short water wire: solvent effects open the gate

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.

Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate

“…Photoacids and photobases, molecular systems that respectively exhibit a profound increase in acidity or basicity, , are ideal molecular tools to study proton transport dynamics on ultrafast time scales using short pulse laser systems. , Proton dissociation to or proton abstraction from water and alcohol solvents − and photoacid–base neutralization reactions − have been investigated with picosecond time-correlated single photon counting, femtosecond UV–vis and UV–IR pump–probe techniques. A special class are so-called bifunctional photoacids and photobases, − where proton-donating and -accepting sites are part of the same molecular system, with a well-defined number of solvent molecules constituting the proton transport pathway between donating and accepting sites. In a recent study ,, we have used 7-hydroxyquinoline (7HQ; see Figure a), which is both a photoacid and a photobase through its hydroxyl proton-donating and quinoline nitrogen proton-accepting functionalities, to explore the proton transport pathways in water/methanol mixtures.…”

“…A special class are so-called bifunctional photoacids and photobases, − where proton-donating and -accepting sites are part of the same molecular system, with a well-defined number of solvent molecules constituting the proton transport pathway between donating and accepting sites. In a recent study ,, we have used 7-hydroxyquinoline (7HQ; see Figure a), which is both a photoacid and a photobase through its hydroxyl proton-donating and quinoline nitrogen proton-accepting functionalities, to explore the proton transport pathways in water/methanol mixtures. We have found that proton abstraction by the quinoline nitrogen is the rate-determining step (N* → C*), followed by hydroxide/methoxide transfer toward the hydroxyl proton-donating site, completing the proton transport (through the C* → Z* step).…”

Proton transfer in a short hydrogen bond caused by solvation shell fluctuations: an ab initio MD and NMR/UV study of an (OHO)− bonded system

“…[72][73][74] These results illustrate the highly complex nature of the reaction coordinate manifold in solution. [81][82][83] In the past such systems have often been used to study nuclear quantum effects, e.g. [76][77][78][79][80] On the other hand, static quantum chemical calculations are not able to deal with the large number of possible transitions, hence the dominance of AIMD as a tool to investigate these processes.…”

“…While the above applies mostly to aqueous solutions, model systems with welldefined proton exchange pathways serve a useful purpose in investigating proton transfer reactions. [81][82][83] In the past such systems have often been used to study nuclear quantum effects, e.g. proton quantum delocalization, which is substantial in systems with low barriers and at low temperatures.…”

A theoretical study on excited state proton transfer in 2‐(2′‐dihydroxyphenyl) benzoxazole

“…Hydrogen bonding interactions between biological chromophores and their surrounding protein and solvent environment affect significantly the photochemical or photophysical pathways of the chromophore and its biological function . A common first step in the dynamics of these systems is the proton transfer process, which plays a crucial role in a large variety of chemical reactions and biological systems .…”