Ab Initio Frequency Calculations of Pyridine Adsorbed on an Adatom Model of a SERS Active Site of a Silver Surface

Ab initio and Urey−Bradley calculations of normal modes were carried out to analyze the frequency shifts observed on the SERS spectrum of pyridine obtained on a Ag surface. A molecular model with a Ag atom attached to the pyridine nitrogen was used in the Urey−Bradley calculation. The frequency shifts in the SERS spectrum were calculated by adding K(Ag−N) and F(Ag· · ·C α ) force constants and by adjusting the ring stretches according to how their bond lengths changed in an ab initio calculation with a (Ag−Py) + model. The ab initio calculations were done with the HF, MP2, and B3LYP methods using the 3-21G, 6-31G, 6-31G*, 6-31G**, and LanL2DZ basis sets. The surface active site was modeled in the calculation by means of Ag 0, Ag + atoms, and a Ag 4 + pyramidal cluster structure. The calculations were evaluated on how well they matched the observed frequencies, the ν(Ag−N) stretch, and the frequency shifts for the SERS spectrum in an electrochemical environment. The calculation with the Ag 4 + model yielded the best results although the (Ag−Py) + model also gives reasonable results. All calculations were consistent with an edge-on interaction between pyridine and the electrode surface and a Ag + species as part of the surface active site.