Normal Raman (NR) and surface enhanced Raman scattering (SERS) spectra of 3-hydroxyflavone (3-HF) have been measured. The SERS spectra are obtained both on an electrode surface and on Ag colloid. Spectral bands are assigned using DFT calculations for the NR, SERS, and FTIR spectra. The IR, normal Raman, and SERS results are compared and in conjunction with a DFT calculation are used to interpret the low-frequency carbonyl stretching vibration in 3-HF. The experimental results support the DFT geometry calculations, which show that an adatom site at the vertex of Ag20 cluster binding with the 3-HF molecular plane tilted at an angle of about 53° to the surface is a low-energy structure. This is consistent with the enhancement of in-plane vibrational modes.

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 Ag 20 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(H 2 O), 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 photooxygenation product, evidence for an additional endo-peroxide intermediate is obtained.

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