Competition between excited state proton and OH(-) transport via a short water wire: solvent effects open the gate.
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.
Physics Department, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.
Gül
G
Bekçioğlu
Christoph
C
Allolio
Maria
M
Ekimova
Erik T J
ET
Nibbering
Daniel
D
Sebastiani
D003198:Computer Simulation; D017665:Hydroxyl Radical / Q000737:chemistry*; D006912:Hydroxyquinolines / Q000737:chemistry* / Q000528:radiation effects; D008027:Light; D044085:Microfluidics / Q000379:methods*; D008956:Models, Chemical; D011522:Protons; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry* / Q000528:radiation effects; D014867:Water / Q000737:chemistry*
D006912:Hydroxyquinolines; D011522:Protons; D012997:Solvents; D014867:Water; D017665:Hydroxyl Radical