THE BONDING OF ACETATE, METHOXY, THIOMETHOXY AND PYRIDINE TO CU SURFACES - A MOLECULAR-ORBITAL STUDY

The bonding of acetate (CH3COO), methoxy (CH3O), thiomethoxy (CH3S) and pyridine (C5H5N) to copper surfaces has been examined employing semi-empirical MO-SCF calculations (INDO/S) and metal clusters of limited size (Cu(n), n = 16 or 18 atoms). CH3COO, CH3O and CH3S behave as electron acceptors when adsorbed. For these species, the chemisorption bond is dominated by the interaction between the LUMO of the adsorbate and the Cu(4s, 4p) bands. The relatively weak C-S bond in CH3Sa makes decomposition to form sulfur adatoms and alkanes a very exothermic process (- DELTA-H = 20 to 30 kcal/mol). In contrast, similar types of decomposition reactions for CH3Oa are almost thermoneutral (as a consequence of a strong C-O bond), and the molecule prefers to decompose forming H2COa and H(a) species. The results of a thermochemical analysis indicate that reactions which involve the cleavage of S-H and/or C-S bonds of alkanethiols are very exothermic on copper. The bonding mechanism of pyridine involves a large charge transfer from the 7a1 and 2b1 orbitals of the molecule into the Cu(4s, 4p) orbitals, and a very small electron transfer from the substrate into the C-N antibonding 3b1 orbital of the adsorbate (pi-backbonding). The fact that Cu is poor at pi-backdonation makes the metal inactive for pyridine decomposition. On the basis of these INDO/S results, the possible UPS spectra of CH3O and CH3S On Cu(111), and of CH3COO and C5H5N on Cu(110) are discussed and compared with experimental results.