Spin uncoupling in surface chemisorption of unsaturated hydrocarbons

Unsaturated hydrocarbons, such as acetylene and ethylene, show strong geometrical distortions when coordinated to transition metals or to surfaces; the bonding is normally analysed in terms of a π-donation - π*-backdonation process. In the present work we use chemisorption of the unsaturated hydrocarbons (ethylene, acetylene, and benzene) on cluster models of the copper (100), (110), and (111) surfaces to demonstrate the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of the adsorbate at the surface. By comparison to the structures of the triplet excited states in the gas phase we demonstrate that these must be considered as the states actually involved in the bonding. This implies a spin-uncoupling in both adsorbate and substrate as part of the chemisorption process or bond formation. In particular, for benzene we identify the quinoid gas phase triplet state as the specific state that is most strongly bound to the Cu(110) substrate; the structure is an inverted boat form. The gas phase antiquinoid triplet state leads to a planar, less strongly bound, chemisorbed state. By explicitly considering the excited state of the adsorbate that corresponds to the bonding state - the ground state for the chemisorbed system - barriers in the chemisorption path are analyzed in terms of avoided crossings between the initial closed-shell singlet state and the bond-prepared excited triplet state, which, together with the substrate, forms an overall singlet. It is argued that this picture with bond-preparation through spin-uncoupling can be very useful to understand and predict reaction paths in heterogeneous catalysis.