Water adsorption on free cobalt cluster cations

Cationic cobalt clusters complexed with water Con+-​H2O (n = 6-​20) are produced through laser ablation and investigated via IR multiple photon dissocn. (IR-​MPD) spectroscopy in the 200-​1700 cm-​1 spectral range. All spectra exhibit a resonance close to the 1595 cm-​1 frequency of the free water bending vibration, indicating that the water mol. remains intact upon adsorption. For n = 6, the frequency of this band is blue-​shifted, but it gradually converges to the free water value with increasing cluster size. In the lower-​frequency range (200-​650 cm-​1) the spectra contain several bands which show a very regular frequency evolution, suggesting that the exact cluster geometry has little effect on the water-​surface interaction. D. functional theory (DFT) calcns. are carried out at the OPBE​/TZP level for three representative sizes (n = 6, 9, 13) and indicate that the vibrations responsible for the resonances correspond to bending and torsional modes between the cluster and water moieties. The potential energy surfaces describing these interactions are very shallow, making the calcd. harmonic frequencies and IR intensities very sensitive to small geometrical perturbations. We conclude that harmonic frequency calcns. on (local) min. structures provide insufficient information for these types of cluster complexes and need to be complemented with calcns. that provide a more extensive sampling of the potential energy surface.