The bonding of acetate, methoxy, thiomethoxy and pyridine to Cu surfaces: a molecular orbital study

Abstract

The bonding of acetate (CH 3COO), methoxy (CH 3O), thiomethoxy (CH 3S) and pyridine (C 5H 5N) 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). CH 3COO, CH 3O and CH 3S 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 CH 3S a makes decomposition to form sulfur adatoms and alkanes a very exothermic process (− ΔH = 20 to 30 kcal/ mol). In contrast, similar types of decomposition reactions for CH 3O a are almost thermoneutral (as a consequence of a strong CO bond), and the molecule prefers to decompose forming H 2CO a 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 7a 1, and 2b 1 orbitals of the molecule into the Cu(4s, 4p) orbitals, and a very small electron transfer from the substrate into the CN antibonding 3b 1 orbital of the adsorbate (π-backbonding). The fact that Cu is poor at π-backdonation makes the metal inactive for pyridine decomposition. On the basis of these INDO/S results, the possible UPS spectra of CH 3O and CH 3S on Cu(111), and of CH 3COO and C 5H 5N on Cu(110) are discussed and compared with experimental results.