Tautomerism of adsorbed thymine on gold electrodes: an in situ surface-enhanced infrared spectroscopy study

Abstract

The adsorption of thymine on gold electrodes has been studied by ATR-SEIRAS in situ spectroscopy at three pH values (1, 7 and 12), in order to analyse the tautomeric equilibrium of deprotonated thymine in the adsorbed state. The anionic form which is deprotonated in nitrogen N1 (N1 tautomer) and the one deprotonated in nitrogen N3 (N3 tautomer) are considered. The spectroscopic results have been interpreted in the light of DFT ab-initio calculations of both tautomeric forms adsorbed on gold surfaces. The basis sets 6-311 + +G(d,p) for C, H, O and N atoms and LANDL2DZ for Au atoms have been used. The electrode surface has been simulated as an unreconstructed Au(111) surface of 19 atoms. The preponderance of each tautomer of adsorbed thymine as a function of pH has been inferred from the analysis of the spectral signals in the 1500–1800 cm−1 wavenumber region related mostly to stretching modes of the carbonyl groups. It has been found that the adsorbed N3 tautomer predominates in the case of adsorption from acid media while the N1 tautomer predominates in the adsorption from basic media but the adsorption of both forms have been distinguished in the experiments from neutral media. Moreover, the plausible changes in the orientation of the two adsorbed tautomers with the electric field have been determined by analysing the relative intensities of the characteristic CH stretching signals in the 2800–3300 cm−1 spectral region. It is proposed that the N3 tautomer undergoes changes in the interaction sites with the metal and in the molecular plane orientation on the electrode as the electric potential is increased, but the N1 tautomer molecule keeps the interaction sites while the molecular plane slightly rotates to facilitate the electrostatic interaction with the field.