Effect of Heteroaromaticity on Adsorption of Pyrazine on the Ge(100)-2x1 Surface

The reaction of pyrazine with the Ge(100)-2x1 surface has been investigated by X-ray photoelectron spectroscopy (XPS) and density functional theory calculations. Results show that pyrazine reacts with the surface through both the nitrogen and carbon atoms of the ring to form N-dative bonds and cycloaddition products, respectively. These products are assigned based on calculated electron core level binding and adsorption energies, as well as experimental XPS results. We find that the product distribution changes as a function of coverage and temperature, and in all cases, both dative bonded and cycloaddition products are present. A temperature dependence analysis shows evidence of molecular desorption as well as changes in the product distribution. We found that the number of nitrogen dative bonds is maximized at low temperature through kinetic trapping, while carbon cycloaddition products are promoted with increasing thermal energy. Coverage dependence analysis shows that as the surface becomes crowded, most of the reactions occur through the nitrogen moiety. The nature of surface-adsorbate bonding is revealed by natural population analysis and energy decomposition analysis. We find strong evidence for the dative bond character from molecule to surface donation of the nitrogen lone pairs and surface to molecule back-donation from Ge-Ge-subsurface bonds. A loss of aromaticity is found for the structure with two dative bonds. The reaction to the cycloaddition product shows signatures of an inverse electron-demand Diels-Alder reaction.