Surface-enhanced Raman spectroscopic study of uracil. The influence of the surface substrate, surface potential, and pH

The Raman and surface-enhanced Raman spectra of uracil have been recorded under a range of experimental conditions and the vibrational spectra of uracil and deprotonated uracil in the condensed phase are predicted by density functional theory (DFT) calculations at the B3LYP/6-31(++)G(d,p) level. Solvation effects are taken into consideration in two different ways: in the context of a self-consistent reaction field of a dielectric continuum and by the explicit addition of two water molecules that can form H-bonds with the C=O and N-H moieties of uracil. On the basis of these calculations combined with normal Raman spectroscopy (NRS), two different tautomers corresponding to N1- and N3-deprotonated uracil are identified in alkaline aqueous solution, the N1-deprotonated species being slightly more common. In the SERS spectra of alkaline uracil solution in a silver sol, contributions from both tautomers are detected. The ratio of the two tautomers depends on the analyte concentration. At submonolayer concentrations (ca. 10(-6) M) both tautomers interact with the silver surface via the respective deprotonated nitrogens adopting tilted orientations. At higher concentrations the competition for adsorption sites leads to a more upright orientation of the adsorbed species and the N3-deprotonated tautomer being favored. DFT calculations at the B3LYP/LanL2DZ level prove that N3-deprotonated uracil is stabilized more by the presence of Ag(+) ions at the metal surface than N1-deprotonated uracil. At neutral pH uracil adsorbs to the silver colloid exclusively in its N-3 deprotonated form. The interaction between uracil and an electrochemically roughened silver electrode is similar to the interaction between uracil and the silver colloid. Spectral changes caused by varying the applied electrode potential are most likely due to the inductive effect of the metal rather than a molecular reorientation at the metal surface.