A Surface-Enhanced Raman Scattering (SERS) spectrum of 4-cyanopyridine (4CNPy) was recorded on silver plasmonic nanoparticles and analyzed by using Density Functional Theory (DFT) calculations. Two simple molecular models of the metal–4CNPy surface complex with a single silver cation or with a neutral dimer (Ag+–4CNPy, Ag2–4CNPy), linked through the two possible interacting sites of 4CNPy (aromatic nitrogen, N, and nitrile group, CN), were considered. The calculated vibrational wavenumbers and intensities of the adsorbate and the isolated species are compared with the experimental Raman and SERS results. The analysis of the DFT predictions and the experimental data indicates that 4CNPy adsorbs preferentially on neutral/charged active sites of the silver nanoparticles through the nitrogen atom of the aromatic ring with a perpendicular orientation.

Vibrational wavenumbers of pyridine adsorbed on a silver electrode have been correlated to the calculated ones from different theoretical approaches based on DFT methods. The vibrational tuning caused by the electrode potential has been simulated by means of pyridine-silver clusters with different densities of charge or, alternatively, under applied external electric fields. Both methodologies predict correctly a qualitative red-shift of the vibrational wavenumbers at negative potentials. As a result, harmonic frequency calculations performed at the B3LYP/LanL2DZ level of theory by using a linear [Ag n Py] q complex model with different densities of charge ( q eff = q/n ) have exhibited the best agreement with the experimental observations although the tuning amplitudes are overestimated. Electric fields calculations are unable to account for subtle details observed in the spectra related to the differentiated chemical nature of the metal-molecule bond at positive or negative potentials with respect to the potential of zero charge of the electrode.

The striking SERS enhancement of the out-of-plane 16b mode of pyridazine is due to resonant metal-to-molecule CT processes and not to the planar orientation of the adsorbate.

Experimental and theoretical calculations confirm, for the first time, the existence of two different kinds of electronic structures of a surface complex formed by a particular molecule bonded to charged metal electrodes, clusters, or nanoparticles. Surface-enhanced Raman scattering (SERS) of cyanine adsorbed on a silver electrode shows three regions, which are selected by the electrode potential and characterized by the differentiated response of the vibrational wavenumbers of the ν(CN) stretching band to the electrode potential. The combination between the experimental SERS and DFT calculations has allowed for relating the three regions to chemisorbed (C-hybrid) and physisorbed (P-hybrid) surface complexes, where cyanide is bonded through the carbon on top of a single silver atom of the surface and to bidentate species, respectively. The electrode potential selects one or another type of electronic structure of the surface complex, which are of different natures and with a differentiated response to the applied potential. The electric potential tunes smoothly the wavenumbers, bond energies, and injected charges of the P-hybrid at more negative potentials than that of the zero charge of the electrode, but the very strong C-hybrid prevents significant changes of these properties at positive excesses of charge. The existence of the dual electronic structure of metal-molecule interfaces might require reinterpreting experiments that are usually discussed by resorting to, for instance, the reorientation of the adsorbate, the formation of complexes with different stoichiometries, the existence of nonequivalent local sites on the surface, or to instrumental artifacts.

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