Comparative performance of theoretical tools in order to quantify the effect of the electric potential on the vibrational wavenumbers and intensities of the SERS of 2‐methylpyrazine adsorbed on a nanostructured silver electrode

Abstract

AbstractThe effect of the electrode potential in surface‐enhanced Raman scattering (SERS) intensities and wavenumbers of 2‐methylpyrazine (2MP) was analyzed on the basis of a resonant charge transfer (CT) mechanism by using a simple theoretical model in which the metallic surface and its charge density were simulated by atomic silver clusters of different size (n) and charge (q), [Agn]q. Two linear silver atoms (n = 2) with zero charge (q = 0) and three linear silver atoms (n = 3) with positive and negative charges (q = ±1) linked to the two nonequivalent aromatic nitrogen atoms in 2MP were taken into account. The wavenumber shifts of the most intense bands and the SERS‐CT spectra of these two types of metal‐adsorbate supermolecule, [Agn‐N1]q and [Agn‐N4]q, were calculated by using a time‐dependent density functional theory (TD‐DFT) method and the independent mode displaced harmonic oscillator (IMDHO) approximation. A comparison of the effect of different levels of calculation, ab initio/DFT, on the predictions from the two theoretical models (isolated adsorbate/supermolecule) is also performed. Only DFT theoretical results of the metal‐adsorbate supermolecule allow to explain the main role of the pair of bands assigned to totally symmetric ring‐stretching 8a,b modes. The 8a vibration is the strongest band at any electrode potential, whereas the 8b mode reaches a maximum enhancement at −0.50 V and then decreases at −0.75 V. This model of a charged metal‐adsorbate interface allows for detecting the presence of a CT mechanism in a SERS record.