Resolving the Structures of AlM ₂ O ₄ ⁺ (M = Fe, Co) through Multireference Methods

Multicenter transition metal clusters often exhibit a dense manifold of low-lying electronic states arising from the interplay of ligand-field splitting and spin coupling. Together with structural and valence isomerism, these factors make vibrational spectral assignments challenging. The infrared photodissociation (IRPD) spectra of AlM 2 O 4 + (M = Fe, Co) have resisted interpretation because the predicted spectra are sensitive to the isomer and its electronic state. Here we settle the assignment by employing multireference methods in a localized active space framework to identify the low-energy structures and their ground-state electronic configurations, followed by vibrational analysis using Kohn–Sham density functional theory. We show that the experimental IRPD spectra arise from the coexistence of two structural isomers, each in a low-spin ground state. Vibrational spectra calculated for the identified ground states reproduce the experimental IRPD features, whereas those of alternative states do not. This assignment is consistent with reported experimental observations, namely the IRPD spectra, the ion mobility distributions, and the absence of methane reactivity. Analysis of the electronic structure reveals that ligand-field effects dominate over spin coupling in shaping both energetics and vibrational spectra of these clusters. These results show the power of multireference methods capable of treating large active spaces, which are often required for binuclear complexes.