Direct observation of size dependent activation of NO on gold clusters.

Vibrational spectra of NO ligands adsorbed on cationic gold clusters are obtained using IR multiphoton dissociation spectroscopy. We observe a strong oscillation of the NO stretching frequency with increasing cluster size that reveals a clear odd-even dependence in the strength of the NO bond. For the corresponding CO complexes, no oscillation is found. This behaviour can be understood on the basis of the character and occupation of the frontier orbitals of the complexes.