Bonding interaction, low-lying states and excited charge-transfer states of pyridine-metal clusters:: Pyridine-Mn (M=Cu, Ag, Au; n=2-4)

The binding interactions between the pyridine and small coinage metal clusters such as copper, silver and gold in different sizes (n = 2-4) have been investigated by a density function theory approach. We obtain the binding energies of these pyridine-metal cluster complexes and analyze the nature of the binding interactions in terms of the metal cluster-dependent molecular orbital properties and binding orientations. The calculated binding energies show a good agreement with the experimental values of the pyridine adsorbed on the metal surfaces. We find that the order in the binding energies between pyridine and then metals clusters is Cu-Au>Ag and the interaction between pyridine and the silver is the weakest among these three metals. The donation of the lone-pair electrons on the nitrogen atom of the pyridine molecule is the major contribution to the binding interaction between pyridine and each metal cluster. The back-donation interaction from the metal atoms to the pi-type anti-bonding orbital is very weak in all these complexes. The time-dependent density functional theory has also been performed to provide information on the relative energies of the low-lying states of each complex with respect to its ground state. In particular, we have explored the excited charge-transfer states, which play an important role in surface-enhanced Raman scattering processes. Based on the calculated molecular properties, we discuss Raman spectroscopy of pyridine adsorbed on rough metal surfaces. (C) 2003 American Institute of Physics.