Infrared photodissociation spectroscopy of (Al 2 O 3 ) 2-5 FeO + : influence of Fe-substitution on small alumina clusters.

The infrared photodissociation spectra of He-tagged (Al 2 O 3 ) n FeO + ( 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 Al 2 FeO 4 + ( 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.