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The differences between alternative approaches for quantifying the complex effect of the electronic structure of charged metal–molecule hybrids in SERS (the so-called chemical enhancement mechanism) are highlighted. The discussion is focussed on the predictions obtained by using different methodological tools for modelling the role of the electrode potential in the SERS relative intensities. Finite electric fields are used in the commented paper for this purpose, but we have found some inconsistences in the results and the method for calculating the Raman intensities is not made sufficiently clear, which prevents the assessment of the results.

DFT calculations predict that plasmon-like excitations in small metal clusters are able to selectively modify the relative intensities of specific SERS bands of adsorbed molecules. These electronic resonances provide new kinds of SERS selection rules which can explain the huge enhancement of mode 9a of pyridine in the spectra recorded at negative electrode potentials.

A comprehensive analysis of the effectiveness of the electrode potential (EV) in tuning the energies of the electronic states (E) of hybrid systems composed by molecules (A) bonded to discrete metal clusters (M) is carried out for the first time by combining experimental SERS data with TDDFT calculations on [Mn-A]q complexes of different sizes (n) and charges (q). It will be demonstrated that the unexplained origin of observed huge energy gains up to G = 5 eV/V involves two different contributions: G = ΔE/ΔEV = SC. The first one is responsible for the effect of a fractional charge (qeff = q/n) on the electronic structure of [Mn-A]q complexes which tunes the CT states through the slopes S of the E vs qeff diagrams. The second contribution (C = Δqeff/ΔEV) is a kind of capacitance that quantifies the ability of the metal to convert the electrode potential EV into the atomic excess of charge qeff. The estimated capacitances of nanometer-size SERS active hot spots are 2 or 3 times larger than those measured fr...

The analysis of photoinduced charge transfer (CT) processes in a particular SERS spectrum is not a trivial task since each system must be studied separately and no universal selection rules can be proposed for this kind of enhancement mechanism. Despite this, we found out in previous studies that the SERS-CT spectra of benzene-like molecules are mainly characterized by the strong enhancement of the totally symmetric 8a ring-stretching vibration and we have used this as a propensity rule to recognize the presence of resonant CT processes in the SERS of these adsorbates. However, it is necessary to cautiously consider the relevant symmetry of the system under study before applying this simplified rule. When the molecule shows a low symmetry (or the relevant symmetry is reduced by the adsorption on the metal as it was demonstrated in the case of pyrimidine), the selective intensification of the 8a mode can be disguised by the redistribution of the CT enhancement with another close vibration in wavenumber, na...

DFT calculations predict two different electronic structures of metal–molecule hybrids which are selected depending on the surface charge. While the metal-to-molecule CT states are very sensitive to the charge, the energies of the reverse molecule-to-metal CT processes are surprisingly not modified at all by the charge of the metal.