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

The bonding of acetate (CH 3 COO), methoxy (CH 3 O), thiomethoxy (CH 3 S) and pyridine (C 5 H 5 N) 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 3 COO, CH 3 O and CH 3 S 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 3 S 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 3 O a are almost thermoneutral (as a consequence of a strong CO bond), and the molecule prefers to decompose forming H 2 CO 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 3 O and CH 3 S on Cu(111), and of CH 3 COO and C 5 H 5 N on Cu(110) are discussed and compared with experimental results.