Valence and Structure Isomerism of Al_2FeO_4~+: Synergy of Spectroscopy and Quantum Chemistry

摘要

We provide spectroscopic and computational evidence for a substantial change in structure and gas phase reactivity of A1_3O_4~+ upon Fe-substitution, which is correctly predicted by multireference (MR) wave function calculations. A1_3O_4~+ exhibits a cone-like structure with a central trivalent O atom (C_(3v) symmetry). The replacement of the Al- by an Fe atom leads to a planar bicyclic frame with a terminal Al-O~(·-)* radical site, accompanied by a change from the Fe~(+Ⅲ)/O~(-Ⅱ) tothe Fe~(+Ⅱ)/O~(-Ⅰ) valence state. The gas phase vibrational spectrum of Al_2FeO_4~+ is exclusively reproduced by the latter structure, which MR wave function calculations correctly identify as the most stable isomer. This isomer of Al_2FeO_4~+ is predicted to be highly reactive with respect to C-H bond activation, very similar to A1_8O_(12)~+ which also features the terminal Al-O~(·-) radical site. Density functional theory, in contrast, predicts a less reactive Al_3O_4~+-like "isomorphous substitution" structure of Al_2FeO_4~+ to be the most stable one, except for functionals with very high admixture of Fock exchange (50%, BHLYP).