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).

中文翻译：

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Al2FeO4+ 的价态和结构异构：光谱学和量子化学的协同作用

我们为 Al3O4+ 在 Fe 取代后的结构和气相反应性发生实质性变化提供了光谱和计算证据，这是通过多参考 (MR) 波函数计算正确预测的。Al3O4+ 呈现锥形结构，具有中心三价 O 原子（C3v 对称性）。用 Fe 原子取代 Al- 导致平面双环框架具有末端 Al-O•- 自由基位点，伴随着从 Fe+III/O-II 到 Fe+II/OI 价态的变化。Al2FeO4+ 的气相振动谱完全由后一种结构再现，MR 波函数计算正确地将其识别为最稳定的异构体。预计这种 Al2FeO4+ 的异构体对 CH 键的活化具有高度反应性，与 Al8O12+ 非常相似，Al8O12+ 也具有末端 Al-O•- 自由基位点。