Surface-enhanced Raman spectroscopic and density functional theory study of adenine adsorption to silver surfaces

The surface-enhanced Raman spectra (SERS) of adenine and three deuterated analogues adsorbed on colloids, electrochemically roughened electrodes, and vacuum deposited island films of silver have been investigated. All normal Raman and SERS bands were assigned to normal modes on the basis of density functional theory (DFT) calculations (B3LYP/6-31(++)G(d,p)) and isotope shifts. Surface selection rules derived from the electromagnetic enhancement model were employed to deduce adenine orientations on the different surfaces. On the colloids, adenine adopts an almost perpendicular orientation interacting with the metal surface via N7 and the exocyclic amino group. On the electrodes, adenine adsorbs in a more tilted orientation while on the island films the tilt is even more pronounced. Interaction with the electrodes takes place through N7 and the amino group, while interaction with the island film may be solely through N7.