Unexpected structural isomers of AlFe2O4+ and AlCo2O4+: vibrational spectroscopy and ion mobility combined with quantum chemistry

Abstract

The structure and reactivity of the mixed metal oxide clusters Al₂MO₄⁺ and AlM₂O₄⁺ (M = Fe, Co), formally obtained by transition metal ion substitution from Al₃O₄⁺, are studied using infrared photodissociation (IRPD) spectroscopy, ion-mobility mass-spectrometry (IM-MS) and quantum chemistry. We use density functional theory (DFT) in combination with global structure optimization to identify low energy structures and to connect them to the IRPD and IM-MS data. Insights into anharmonic and temperature effects are obtained from machine learning-based molecular dynamics simulations. While all metal ions are equal in the cone-shaped structure of M₃O₄⁺, the mixed metal oxide clusters attain different, more stable structures, in which the metal ions are either in different oxidation states (Al₂MO₄⁺) or have different coordination numbers (AlM₂O₄⁺). The present results illustrate that different DFT functionals may accurately describe local minimum structures, but reliable relative energies of isomers with differently coordinated transition metal ions require multi-reference wavefunction calculations.