INTERACTION OF ACETONITRILE WITH PLATINUM (111) - MORE PROPERTIES OF THE ETA-2(C,N) STATE AND NEW SPECIES IN THE SUBMONOLAYER

The interaction of CH3CN on Pt(111) was studied with infrared reflection absorption spectroscopy (IRRAS), temperature programmed desorption (TPD) and work function change (DELTAphi). Submonolayer, monolayer and multilayer states are distinguished. Measurements at submonolayer coverages are consistent with an orientation of adsorbed acetonitrile which is parallel to the surface. No infrared absorption attributable to the CN bond at around 2200 cm-1 or 1600 cm-1 could be detected by IRRAS, consistent with an eta2(C,N) state for the monolayer, as suggested by Sexton and Avery. In addition, IRRAS data showed that there was a red shift of the C-H stretches of the methyl group with the absence of methyl bends. Agostic bonding involving the beta-hydrogen is proposed to explain this observation. We denote this state as eta2*(C,N) to emphasize the involvement of the beta-hydrogen in chemisorption. For monolayer coverage, IR activity for all bands present in the submonolayer spectrum decreased in intensity. The magnitude of this effect seems too large to be attributed solely to screening by neighbouring dipoles. The multilayer showed IR bands in good agreement with those in the vapour phase spectrum. When the crystal was annealed to remove the multilayer and part of the monolayer, IRRAS detected two species with the CN dipole having a component normal to the surface. These are the terminal and bridge bonded acetonitrile adsorbed via the nitrogen atom. Implications for chemical vapour deposition using cyanide containing precursors are discussed.