High temperature phase of the DNA base thymine on Cu(1 1 0): a resonance delocalised bonding system

The high temperature, deprotonated phase of the DNA base thymine (C5H6N2O2), created at 558 K on a Cu(1 1 0) surface, has been investigated using reflection absorption infrared spectroscopy (RAIRS). The RAIR spectra show absence of ν(CO) stretching vibrations expected above 1620 cm−1, indicating that the adsorbed anionic species cannot be described in terms of a diketone form. Instead, the spectrum is dominated by new absorption features between 1600–1400 cm−1 which correspond closely to those observed for model compounds containing the N(3) deprotonated tautomer of thymine and possessing a resonance delocalised [OC(2)N(3)C(4)O]– bond system. It is, therefore, proposed that the anionic thymine adsorbed on Cu(1 1 0) retains this delocalised electronic structure via which the interaction with the surface is mediated. RAIRS dipole selection rules allow an orientation of the adsorbed species to be suggested in which the N(3) and C(2)O functionalities are in close proximity to the metal, producing a geometry that we believe would naturally lend itself to intermolecular hydrogen-bonded assembly at the surface.