The COSY webinars will be organized every three months (each webinar will consist of one lecture):

• March 25th, 2024
• September 18th, 2024

The 2nd Webinar:

Moderator: Kaido Sillar (University of Tartu)

Speaker: Joachim Sauer (Institut für Chemie, Humboldt-Universität zu Berlin, Germany)

Effect of Fe substitution in aluminium oxide gas phase clusters – a challenge for quantum chemistry

We study gas phase clusters as models for solid catalysts, “the active site in isolation”, and are interested in how the structure and reactivity of gas phase clusters differ from those of the bulk phase with the same composition. We use density functional theory (DFT) to perform global structure optimizations (genetic algorithm). To verify the global minimum structures, we calculate IR spectra and compare them with the ones obtained for mass-selected clusters by photodissociation (IRPD) and multiple photon dissociation (IRMPD) spectroscopy (cooperation with Knut Asmis, Leipzig).

Here, we focus on the effect of Fe substitution on the structure and reactivity of the aluminium oxide clusters Al₃O₄⁺ [1] and Al₈O₁₂⁺. For Al₃O₄⁺, the spectra predicted by DFT indicate that Fe substitution leads to a structural change which is driven by a transition from the Fe^+III/O^-II to the Fe^+II/O^-I oxidation states. DFT does not get the relative stabilities right and multireference (MR) calculations are needed. Even if the number of atoms in this cluster is not large, the MR calculations are difficult to converge with the size of the active space (number of orbitals on the O atoms in addition to the d orbitals on Fe). The relative stabilities obtained with MRCI and NEVPT2 are 52 ± 17 and 69 ± 14 kJ/mol, respectively.

In contrast to Al₃O₄⁺, Fe substitution is isomorphous in Al₈O₁₂⁺ and a change of the oxidation states from Fe^+III/O^-I to Fe^+IV/O^-II converts the reactive terminal Al−O^•− bond into a non-reactive terminal Fe=O bond. While the DFT predictions of the IR spectrum agree with experiment for the singlet and triplet spin states, they are not the lowest energy spin states regardless the functional used (PBE performs best here). MR calculations are even more challenging because of the larger number of orbitals on O atoms that would be needed to reach convergence.

[1] Müller, F.; Stückrath, J. B.; Bischoff, F. A.; Gagliardi, L.; Sauer, J.; Debnath, S.; Jorewitz, M.; Asmis, K. R. Valence and Structure Isomerism of Al₂FeO₄⁺: Synergy of Spectroscopy and Quantum Chemistry. J. Am. Chem. Soc. 2020, 142, 18050-18059.

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