Ab initio interpretation of sum frequency generation spectra of 4-cyanopyridine adsorbed on Au(1 1 1) Original Research Article

The sum frequency generation spectroscopy allows the in situ vibrational study of 4-cyanopyridine (4-CP) adsorbed on a gold electrode. Our (DFT, MP2, MP2 + SQM) calculations permit to propose: (1) the whole vibrational structure of the 4-CP (vibrational frequencies, infrared and Raman intensities) and (2) the interpretation of the adsorption phenomenon. The Au(1 1 1) surface has been modelled by a cluster composed of 28 Au atoms. Our results show that the most stable adsorption configuration is the vertical one when the 4-CP is linking on-top (σ axial bond, via N of the pyridine ring). Moreover, the introduction of the electric field in the calculations confirms the observation of the spectra obtained by electrochemical potential scanning: (i) At negative values, the 4-CP is in its stable adsorption geometry; (ii) then, it turns over to a flat configuration (π bond); (iii) at large positive values of the potential, the 4-CP is still vertically linked (σ axial bond via N of the cyano group).