Electron--Proton Decoupling in Excited-State Hydrogen Atom Transfer in the Gas Phase

TL;DR

This study reveals that in gas-phase excited-state hydrogen transfer, electron and proton transfers occur independently, with electron transfer happening much faster than proton transfer, providing new insights into photochemical processes in biomolecules.

Contribution

It demonstrates electron-proton decoupling in gas-phase ESHT using time-resolved spectroscopy, a novel observation in this context.

Findings

Electron transfer occurs in less than 3 picoseconds.

Proton transfer takes about 15 picoseconds.

Electron and proton transfers are decoupled in gas-phase ESHT.

Abstract

Hydrogen-release by photoexcitation, excited-state- hydrogen-transfer (ESHT), is one of the important photo- chemical processes that occur in aromatic acids and is responsible for photoprotection of biomolecules. The mecha- nism is described by conversion of the initial state to a charge- separated state along the O(N)-H bond elongation, leading to dissociation. Thus ESHT is not a simple H-atom transfer in which a proton and a 1s electron move together. Here we show that the electron-transfer and the proton-motion are decoupled in gas-phase ESHT. We monitor electron and proton transfer independently by picosecond time-resolved near-infrared and infrared spectroscopy for isolated phenol--(ammonia)5, a benchmark molecular cluster. Electron transfer from phenol to ammonia occurred in less than 3 picoseconds, while the overall H-atom transfer took 15 picoseconds. The observed electron-proton decoupling will allow for a deeper understanding and control of of photochemistry in biomolecules