Molecular orbital studies of the adsorption of methyl, methylene, and methyne on rhodium(111) and nickel(111) surfaces

The adsorption of CH3, CH2, and CH is studied on Rh(111) and Ni(111) with the atom superposition and electron delocalization (ASED) and EHMO methods. Results are analyzed by calcg. the local d. of states (LDOS) and bond order overlap populations. On Rh(111), CH is adsorbed on threefold sites, CH2 on twofold sites and CH3 on onefold sites in order to restore missing C-H bonds. The height of CHx to the metal surface decreases with decreasing hydrogen content x, while the adsorption energy increases. Adsorption of CH3 on Rh(111) is studied in detail. CH3 bonds on the metal surface mainly via s type interactions of the n CH3 orbital with surface metal atoms of the same symmetry. In case of the onefold adsorption, the HOMO (n CH3) has a large interaction with metal s, pz, and dz2 orbitals. For twofold adsorption it interacts mainly with the sym., s, pz, and dxz metal group orbitals. Interactions of surface metal orbitals with the p and p* orbitals are weak. Preliminary results are presented for the coupling reaction of coadsorbed CH2 and CH3. A strong repulsion due to steric interaction of the hydrogen atoms is found when the carbon-carbon distance is decreased. As a result the direct coupling reaction of CH3 and CH2 does not seem to be a suitable reaction path for the C-C coupling reaction. CH3 adsorption on Ni(111) was also analyzed. With parameters implying a large spatial extension of the d orbitals, CH3 was found to adsorb on onefold sites. Decreasing the spatial extension of the Ni d orbitals causes a shift to multiply bonding adsorption sites. This illustrates the subtle balance between the interaction with the s valence electrons that favor high coordination sites and the interaction with the d valence electrons favoring low coordination sites.