Abstract
Reverse proton transfer (RPT) of 3-hydroxyflavone (3-HF) from the tautomer ground state to normal ground state has been studied by time-resolved surface enhanced Raman scattering (TRSERS) in aqueous solution in the presence of oxygen. The tautomer ground state of 3-HF is formed by an exciting 337 nm nitrogen laser pulse and monitored by a 488 nm Ar+ laser with gated photodiode array detection. B3LYP DFT calculations with 3-HF bound to the apex adatom of a Ag20 cluster model show that a tilted orientation of the molecule with respect to the surface site is reasonable and is consistent with the SERS spectral characteristics. The geometry optimized structures also show that a cyclic monohyrate, 3-HF(H2O), species is stable from which biprotonic PT could take place. By studying the changes of some transient band intensities with time, the lifetime of tautomer 3-HF on a Ag electrode is determined to be 1.4 μs. Besides the photoinduced photo-oxygenation product, evidence for an additional endo-peroxide intermediate is obtained.
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