Valence and Structure Isomerism of Al<sub>2</sub>FeO<sub>4</sub><sup>+</sup>: Synergy of Spectroscopy and Quantum Chemistry
Fabian Müller, Julius B. Stückrath, Florian A. Bischoff, Laura Gagliardi, Joachim Sauer, Sreekanta Debnath, Marcel Jorewitz, Knut R. Asmis
Abstract
We provide spectroscopic and computational evidence for a substantial change in structure and gas phase reactivity of Al3O4+ upon Fe-substitution, which is correctly predicted by multireference (MR) wave function calculations. Al3O4+ exhibits a cone-like structure with a central trivalent O atom (C3v 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 to the Fe+II/O–I valence state. The gas phase vibrational spectrum of Al2FeO4+ is exclusively reproduced by the latter structure, which MR wave function calculations correctly identify as the most stable isomer. This isomer of Al2FeO4+ is predicted to be highly reactive with respect to C–H bond activation, very similar to Al8O12+ which also features the terminal Al–O•– radical site. Density functional theory, in contrast, predicts a less reactive Al3O4+-like “isomorphous substitution” structure of Al2FeO4+ to be the most stable one, except for functionals with very high admixture of Fock exchange (50%, BHLYP).