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.

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 potential energy surfaces of the ground and valence excited states of both 3H-diazirine and diazomethane have been studied computationally by mean of the CASSCF method in conjunction with the cc-pVTZ basis set. The energies of the critical points found on such surfaces have been recomputed at the CASPT2/cc-pVTZ level. Additionally, ab initio direct dynamic trajectory calculations have been carried out on the S(1) and S(2) surfaces, starting each trajectory run at the region dominated by the conformational molecular rearrangement of diazomethane. It is found that both isomers are interconnected along a C(s)() reaction coordinate on each potential surface. Radiationless deactivation of the corresponding S(1) state of each isomer occurs through the same point on the surface, an S(1)/S(0) conical intersection. Thereafter, the system has enough energy to surmount the barrier which leads to dissociation products (CH(2) + N(2)) on S(0) state. Therefore, photoexcitation to S(1) state of either diazirine of diazomethane produces methylene in its lower singlet state on a very short time scale (ca. 100 fs). Furthermore, both isomers can generate excited singlet carbene when they are excited onto the S(2) surface; in this case, they lose the activation energy passing through another common S(2)/S(1) conical intersection and then proceed to dissociation into carbene and N(2) on the S(1) surface. For the special case of methylene, it rapidly experiences deexcitation to S(0) state.

A general model concerning the role of the electrode potential on the charge-transfer enhancement mechanism of SERS is proposed, justifying the existence of a parameter β which reduces the effectiveness of the electrode potential in order to shift the excited charge-transfer level. This explains the observed deviation of β from the unity, and allows for relating the intensity/electrode-potential (SERS−CT) profiles of similar adsorbates with the respective electron affinities. The SERS of pyridine, pyrazine and methylpyrazines have been recorded on silver, being found that the electrode potential in the maximum of the SERS−CT profiles shifts toward more negative values as the number of methylsubstituents attached to the pyrazine ring increases. This result has been explained on the basis of the relative stability of the radical anions given that a correlation between those shifts and the calculated ab initio vertical transition energies between the neutral molecules and their respective radical anions can be established. This confirms the participation of a resonant charge-transfer mechanism in the SERS as well as the usefulness of this technique to gain insight into the electronic properties of the doublet states of the here studied molecules.