Bonding and coupling of C1 fragments on metal surfaces

The bonding and reactivity of CH3, CH2, and CH fragments to Ti (0001), Cr(110), and Co(0001) metal surfaces is examined with extended Huckel band calculations on two dimensional slabs of metal and adsorbate. A local chemical viewpoint is sought through fragment analyses, decompositions of the density of states and overlap population studies. All fragments tend to restore their missing C-H bonds when bound to these surfaces-CH3 prefers the on-top, CH2 the bridging and CH the capping geometry. CH3 anchors more strongly to the on-top site of a metal surface of high d band filling since the antibonding feature at the top of the d band that results from sigma-bonding destabilizes sites of higher coordination. The mobility of these fragments is reduced on metal surfaces of higher d band filling. The mobility patterns of CH3, CH2, CH are examined. On the way to products there are barriers to migration on the surface, a proximity or crowding effect which makes it costly for two fragments to approach on the surface, and a barrier, small or large, to their reaction with each other. When two C1 fragments couple, the C-C sigma* orbital rises from below the Fermi level. It is initially filled, then empties as the reaction proceeds. Hence the lower the Fermi level(for metals at the right side of the transition series), the smaller the reaction barrier.