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· · ·Cα) 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 Ag0, Ag+ atoms, and a Ag4 + 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 Ag4 + 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.

6-Mercaptopurine, 6MP, is a sulfur analogue of adenine which is commonly used as an antitumor agent. The coordination chemistry of 6MP with metals is of interest because the drug contains a number of electronically versatile binding sites. In this study the interaction between 6MP and a silver electrode surface was investigated using surface-enhanced Raman spectroscopy (SERS). High-quality solution Raman spectra of 6MP along with surface-enhanced Raman scattering spectra of 6MP adsorbed on a roughened silver electrode surface are presented. Urey−Bradley force field and semiempirical calculations with the PM3 method were used to generate vibrational frequencies for 6MP based on a specific set of possible molecular interaction sites. An error analysis of the calculated frequencies and the experimentally determined SERS frequencies allowed for the identification of the experimental data which most closely matched the calculated SERS data. It was concluded that 6MP attaches head-on through the N1 atom when the molecule is adsorbed onto a silver electrode surface.