RAIRS investigations on the orientation and intermolecular interactions of adenine on Cu(1 1 0)

The adsorption of the DNA base adenine (C5N5H5) on Cu(1 1 0) has been investigated as a function of coverage and temperature using reflection absorption infrared spectroscopy. These data provide important information on the nature of the local adsorbed complex and the intermolecular lateral interactions that come into play at high coverages. The RAIR spectra are consistent with an adsorption geometry in which the molecular ring is substantially tilted from the surface plane and in which the two amino hydrogens are equidistant from the surface, thus rationalising the appearance of a very strong βNH2 scissoring mode, along with the activity of in-plane vibrational modes and the observation of the symmetric νsymNH2 stretch, but not the asymmetric νasymNH2 stretch. In addition, coordination to the metal surface is proposed to occur at the N(9) position with a possible additional interaction through the N(3) position, both of which are the favoured coordination points in the metal complexes of adenine. This is a strong interaction and leads to a highly stable adsorbed layer. Our data also provide the first direct evidence of hydrogen bonding in the adlayer as coverage is increased, attributed to interactions between the amino group of one molecule and the N(1) and N(7) positions of a neighbouring species. When adsorption is carried out at room temperature, a very heterogeneous adlayer is created in which a diversity of molecular aggregates co-exist. However, upon annealing, a more ordered hydrogen bonded adlayer is formed in which one type of hydrogen bonding assembly is preferred. Finally, we propose that the hydrogen bonded assemblies created at a surface probably involve bent hydrogen bonds which arise from a compromise between the strong molecule-metal interactions that orientate the molecule and weaker lateral hydrogen bonding interactions that dictate the two-dimensional architecture.