Modulating Bond Lengths via Backdonation: A First-Principles Investigation of a Quinonoid Zwitterion Adsorbed to Coinage Metal Surfaces

First-principles calculations reveal that upon adsorption to the Cu(111) surface, the C-C single bonds within the p-benzoquinonemonoimine zwitterion (ZI) contract by about 6%. A detailed analysis reveals that the bond shortening is primarily a result of backdonation from Cu orbitals of s and d symmetry to the lowest unoccupied orbital (LUMO) of the ZI. This LUMO is π∗-antibonding across the molecule and π-bonding across the C-C bond that shortens. We illustrate that the level alignment between the Fermi level of the surface and the frontier molecular orbitals of the ZI, the topology of the LUMO, and the distance between the substrate and the adsorbate are important factors enabling bond strengthening via backdonation. An extended transition state-natural orbitals for chemical valence (ETS-NOCV) analysis is applied to molecular models for this system, and it confirms that the surface → LUMO backdonation on Cu(111) is larger than on Ag(111) and Au(111). (Chemical Equation Presented).