123rd General Meeting of the KCS

Type Symposium
Area Current Advances in Reaction Dynamics
Room No. Room 305+306
Time THU 16:35-17:00
Code PHYS1-3
Subject Ionic effects on excited state proton transfer reaction of Coumarin 183 in aqueous ionic solutions
Authors Joonyoung F. Joung, Sangin Kim, Sungnam Park*
Department of Chemistry, Korea University, Korea
Abstract A photoacid is an aromatic compound that has a lower pKa in its electronic excited state than its ground state and has been used as a model system to study the excited-state proton transfer (ESPT) reactions and proton transfer processes. Although proton dissociation (PD) reactions of weak acids and proton transfer (PT) processes in aqueous solutions are strongly influenced by dissolved ions, a detailed molecular picture that describes how ions affect the rates of PD and PT processes is still missing. Here, we utilize time-resolved fluorescence spectroscopy combined with density functional theory (DFT) calculations to investigate the ionic effect on the ESPT reaction of Coumarin 183 (C183) in aqueous ionic solutions. The local hydrogen bond structure of C183 in the ionic hydration shell is strongly influenced by cations. The activation energy for the ESPT of C183 increases as the charge density of cations is increased. Most importantly, quantum chemical calculations indicate that the proton’s positive charge in the transition state is delocalized through a hydrogen-bonded water channel and is more destabilized as the charge density of cations is increased, leading to a higher activation energy for the ESPT. Finally, the ionic effects on the equilibrium (pKa and pKa*) and kinetic (Ea and KIE) parameters of C183 are found to be well-explained by the free-energy reactivity relation. Our experimental and DFT calculation results provide a molecular level understanding of cationic effects on the equilibria and ESPT kinetics of photoacids in aqueous ionic solutions.
E-mail spark8@korea.ac.kr