Structures of Rhodium Oxide Cluster Cations Rh7Om+ (m = 4–7, 12, 14) Revealed by Infrared Multiple Photon Dissociation Spectroscopy

Abstract

Vibrational spectra of Rh7Om+ (m = 4–7, 12, 14) were measured in the 300–1300 cm–1 range via infrared multiple photon dissociation (IRMPD) spectroscopy. For the oxygen poor cluster sizes, Rh7Om+ (m = 4–7), IRMPD spectra were recorded through photodissociation of Rh7Om+–Ar complexes. IR spectra for Rh7Om+ (m = 12, 14) were recorded via the release of an O2 molecule from Rh7Om+ producing Rh7Om–2+; no O2 loss was observed from Rh7Om+ (m = 4–7, 8, 10). By comparison with the calculated vibrational spectra of several stable isomers obtained using density functional theory, these IR spectra are assigned to geometrical structures. For m = 4–7, all O atoms are bound to Rh atoms only, with a transition of the Rh core from capped octahedral to capped prismatic between m = 5 and m = 6. For the oxygen-rich Rh7O12+ and Rh7O14+, molecular O2 is adsorbed on a bridge site between two Rh atoms. The frequencies of the bands observed bands signal that the O2 molecules are activated, indicating that rhodium oxide clusters with Rh2O3 composition are still capable to donate electrons to activate O–O bonds in adsorbed O2.