This article reports on a project in which reverse proton transfer (RPT) of 3-hydroxyflavone (3-HF) from the tautomer ground state to normal ground state was examined by time-resolved surface enhanced Raman scattering (TRSERS) in aqueous solution in the presence of oxygen.
The tautomer ground state of 3-HF was 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 showed 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 showed 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 was determined to be 1.4 μs. Besides the photoinduced photo-oxygenation product, evidence for an additional endo-peroxide intermediate was obtained. (publisher abstract modified)