INVESTIGATION OF THE VIBRATIONAL PROPERTIES OF CN- ON A PT ELECTRODE BY IN-SITU VIS-IR SUM-FREQUENCY GENERATION AND FUNCTIONAL DENSITY CALCULATIONS

The vibrational properties of CN- pseudohalide ions adsorbed on a Pt(1 10) single-crystal electrode in aqueous neutral solution are studied u sing in situ VIS-IR sum frequency generation (SFG) and compared to the oretical results obtained from density functional theory (DFT) calcula tions on PtNC- and PtCN- molecular ions. As observed previously on a p olycrystalline electrode, the adsorption behaviour of both ions depend s drastically on the electrode potential and on the immersion potentia l in a CN--containing solution. When the electrode is immersed at nega tive potential, two absorption bands are detected in the spectral rang e of the CN- stretching vibration modes on a surface site. The first r esonance at 2070 cm(-1) is already observed at -1.4 V (Ag/AgCl), in th e hydrogen evolution potential region, with an initial potential tunin g rate of 20 cm(-1). A second narrow resonance, peaking at 2150 cm(-1) , starts growing at -0.6 V (Ag/AgCl). The occurrence of two resonances , corresponding to N-bound and C-bound CN- species, is clearly evidenc ed for a single-crystal electrode, and appears as a genuine property o f the system, not a side effect induced by surface defects. The polycr ystalline structure and the defects influence the shape, the position and the width of the resonances, not the basic spectroscopic propertie s of the system. This result is further supported by new density funct ional calculations of the vibrational properties of the PtCN- and PtNC - molecular ions.