Valence and Structure Isomerism of Al₂FeO₄⁺: Synergy of Spectroscopy and Quantum Chemistry

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).本研究提供光谱学与计算证据，表明Fe取代后Al₃O₄⁺的结构与气相反应性发生显著变化，该结果可通过多参考（MR）波函数计算得到准确预测。Al₃O₄⁺呈现类锥结构，中心为三价氧原子，具有C3v对称性。将其中一个Al原子替换为Fe原子后，体系变为带有末端Al–O•–自由基位点的平面双环骨架，同时价态从Fe+III/O–II转变为Fe+II/O–I。Al₂FeO₄⁺的气相振动光谱仅可通过后者结构得到完美重现，而多参考波函数计算可准确将该结构判定为最稳定的异构体。该Al₂FeO₄⁺异构体被预测在C-H键活化反应中具有极高活性，这与同样带有末端Al–O•–自由基位点的Al₈O₁₂⁺极为相似。与之相反，密度泛函理论（Density Functional Theory, DFT）通常会将Al₂FeO₄⁺的、反应性更低的类Al₃O₄⁺"同晶取代"结构预测为最稳定构型，仅当泛函包含极高比例的福克交换（Fock Exchange，50%，如BHLYP泛函）时例外。