Infrared photodissociation spectroscopy of (Al2O3)2–5FeO+: influence of Fe-substitution on small alumina clusters

Abstract

The infrared photodissociation spectra of He-tagged (Al₂O₃)_nFeO⁺ (n = 2–5), are reported in the Al–O and Fe–O stretching and bending spectral region (430–1200 cm⁻¹) and assigned based on calculated harmonic IR spectra from density functional theory (DFT). The substitution of Fe for an Al center occurs preferentially at 3-fold oxygen coordination sites located at the cluster rim and with the Fe atom in the +III oxidation state. The accompanying elongation of metal oxygen bonds leaves the Al–O network structure nearly unperturbed (isomorphous substitution). Contrary to the Al₂FeO₄⁺ (n = 1), valence isomerism is not observed, which is attributed to a smaller M:O ratio (M = Al, Fe) and consequently decreasing electron affinities with increasing cluster size.