Multiple-angle-of-incidence polarization infrared reflection-absorption spectroscopy (MAI-PIRRAS) for investigation of 6-Mercaptopurine SAMs on smooth silver surface

Multiple-angle-of-incidence polarization infrared reflection-absorption spectroscopy (MAI-PIRRAS) has been used to study the adsorption mode and structural properties of 6-Mercaptopurine (6MP) self-assembled monolayers (SAMs) on smooth silver surface. The in-plane vibrations of the 6MP SAMs show some enhancement compared with the spectrum of the solid powder of 6MP, while the out-of-plane vibrations are very weak. Assignments of the observed bands by DFT calculations show that both the thiol form and the tautomer of 6MP exist in the SAMs and adopt the S atom and the N7 or N1 atom anchoring the silver surface. In order to eliminate the influence of the angle of the incidence and obtain direct influence of the orientation of the 6MP to the spectral intensity, the adsorption intensity of the SAMs is multiplied by cos θ to obtain corrected spectra. The adsorption intensity attains to the maximum at 35–40° other than near the grazing angle. The characteristics of the spectra suggest that the SAMs of 6MP are arranged orderly in a parallel fashion with a dihedral angle of 35–40° between the ring plane and the silver surface.