Ab initio interpretation of sum frequency generation spectra of 4-cyanopyridine adsorbed on Au(111)

The sum frequency generation spectroscopy allows the in situ vibrational st udy of 4-cyanopyridine (4-CP) adsorbed on a gold electrode. Our (DFT, MP2, MP2 + SQM) calculations permit to propose: (1) the whole vibrational struct ure of the 4-CP (vibrational frequencies, infrared and Raman intensities) a nd (2) the interpretation of the adsorption phenomenon. The Au(111) surface has been modelled by a cluster composed of 28 Au atoms. Our results show t hat the most stable adsorption configuration is the vertical one when the 4 -CP is linking on-top (sigma axial bond, via N of the pyridine ring). Moreo ver, the introduction of the electric field in the calculations confirms th e observation of the spectra obtained by electrochemical potential scanning : (i) At negative values, the 4-CP is in its stable adsorption geometry; (i i) then, it turns over to a flat configuration (pi bond); (iii) at large po sitive values of the potential, the 4-CP is still vertically linked (sigma axial bond via N of the cyano group). (C) 2001 Elsevier Science B.V. All ri ghts reserved.