The surface-enhanced Raman scattering (SERS) spectra of pyridine have been analyzed on the basis of a resonant charge transfer (CT) mechanism. The most intense bands recorded in the SERS at negative electrode potentials correspond with the normal modes that connect the equilibrium geometries of the neutral molecule and the corresponding radical anion. These normal modes have been characterized according to our previously proposed method by calculating the ab initio geometries of the molecule and its anion and then building the transformation ΔQ = L -1ΔR, where the vector ΔQ contains those vibrations active in SERS-CT via Franck−Condon factors, thereby providing semiquantitative selection rules for this enhancement mechanism.

Articles you may be interested inPhoton-driven charge transfer and Herzberg-Teller vibronic coupling mechanism in surface-enhanced Raman scattering of p-aminothiophenol adsorbed on coinage metal surfaces: A density functional theory study A charge-transfer surface enhanced Raman scattering model from time-dependent density functional theory calculations on a Ag 10 -pyridine complex Surface enhanced Raman scattering from a single molecule adsorbed on a metal particle aggregate: A theoretical study Complete analysis of the surface-enhanced Raman scattering of pyrazine on the silver electrode on the basis of a resonant charge transfer mechanism involving three states Two-dimensional localization of adsorbate/substrate charge-transfer excited states of molecules adsorbed on metal surfaces Surfaced-enhanced Ramon scattering ͑SERS͒ spectra of pyrazine are analyzed on the basis of the properties of the electronic states of the metal-adsorbate surface complex. Ab initio CIS calculations have been carried out for the Ag 2 -pyrazine complex, which have enabled us to find two excited singlets, namely CT 0 ; 1 B 1 and CT 1 ; 1 A 2 , with properties quite similar to those of the pyrazine radical anion in its electronic 2 B 3u and 2 A u states, respectively, and with energies falling in the range of the exciting photons usually employed in Raman spectroscopy. SERS spectra of pyrazine are compatible with a resonance Raman enhancement mechanism involving electronic transitions between the ground state S 0 ; 1 A 1 and both CT levels of the surface complex.

SERS spectra of pyrazine on silver electrode have been recorded and analyzed, assuming a charge transfer effect and using selection rules analogous to those of resonance Raman. With the aim of predicting the effect of this mechanism on the selective enhancement of fundamentals, a method has been proposed based on an analysis of the results of geometry optimizations carried out by ab initio calculations. The strongest SERS bands coincide with those assigned to the normal modes connecting the equilibrium geometries of the neutral molecule and the radical anion. These results support the presence of a charge transfer process from the metal to the adsorbate in the SERS spectra of pyrazine where a significant enhancement of vibrations 8a, 9a, 1, and 6a can be observed. The prediction capability of the analysis proposed here has been checked with published resonance Raman spectra of pyrazine and pyrazine-d 4 . In all of the cases the strongest bands in the spectra are directly related to the largest ∆Q values obtained through the transformation ∆Q ) L -1 ∆R.

A new general procedure to interpret surface-enhanced Raman scattering ͑SERS͒ spectra has been developed in order to clarify the controversy concerning the relevant enhancement mechanism of this type of spectra. The analysis consists of detecting the presence of the charge transfer ͑CT͒ enhancement mechanism by correlating the most enhanced SERS bands with the ab initio calculated geometries (⌬Q) and vibrational frequencies (⌬v) of the isolated molecule and its radical anion. This CT mechanism is assumed to be identical to that of resonance Raman between the electronic ground state of the metal-adsorbate complex and charge transfer excited states. We consider that these excited states arise when one electron is transferred from the metal to pyrazine. For this reason, they have been labeled from the point of view of pyrazine on the basis on the symmetry of the doublet states of its radical anion. The SERS spectra of pyrazine recorded on silver surface at several electrode potentials have been analyzed on the basis of the Franck-Condon and Herzberg-Teller contributions related to 2 B 3u-1 A g and 2 A u-1 A g transitions. A great deal of experimental facts related to the relative enhancement of in-plane A g , B 3g , and B 1u modes as well as out-of-plane B 3u , B 2g , and A u vibrations has been explained. Likewise, it is possible to account for the observation of Raman inactive fundamentals as well as the influence of the forbidden 2 A u-1 A g transition on the SERS spectra if the symmetry of the metal-adsorbate complex and the nonplanarity of the 2 A u state are taken into account. All the results point out that the CT mechanism is mainly responsible for the SERS features of this molecule studied here.

The SERS spectra of 3,5-dimethylpyridine recorded at di †erent electrode potentials were analysed. Their most characteristic feature is the enhancement of the band assigned to mode 8a as the electrode potential becomes more negative. This behaviour is di †erent to that observed in the SERS spectra of pyridine where all four modes 6a, 1, 9a and 8a are enhanced in spite of the fact that both molecules belong to the same molecular point group. These di †erences were explained on the basis of a resonant charge transfer mechanism (CT). The band assigned to the fundamental was used to estimate the electromagnetic mechanism contributions to SERS intensities, m s (CH 3 ) which allowed the direct correlation of the theoretical SERS-CT intensities calculated on the basis of PeticolasÏ equation with experimental results.