Unexpected structural isomers of AlFeO and AlCoO : vibrational spectroscopy and ion mobility combined with quantum chemistry.

The structure and reactivity of the mixed metal oxide clusters AlMO and AlMO (M = Fe, Co), formally obtained by transition metal ion substitution from AlO , 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 MO , the mixed metal oxide clusters attain different, more stable structures, in which the metal ions are either in different oxidation states (AlMO ) or have different coordination numbers (AlMO ). 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.