Geometric Structures and Magnetic Interactions in Small Chromium Oxide Clusters

The physical and chemical properties of transition metal oxide particles result from the subtle interplay between atomic ordering and electronic structure, the latter being determined by a complex interaction between the partially filled d subshell of the transition metal atoms and the oxygen 2p orbitals. In this article, the geometric ground state structures of several experimentally synthesized cationic chromium oxide clusters Cr_mO_n ⁺ (m = 2, 3, 4; n ≤ m) are characterized through infrared photodissociation spectroscopy on cluster-rare gas atom complexes in combination with quantum chemical calculations. Computational analysis of the electronic and magnetic properties of the identified isomers demonstrated that the magnetic configuration of the clusters varies with the size and oxidation state. Superexchange interaction causes ferromagnetic coupling in Cr₂O₂ ⁺ and Cr₃O₃ ⁺, while 3d-3d bonding-like interaction between two chromium atoms underlies ferrimagnetic behavior in Cr₃O⁺, Cr₃O₂ ⁺, and Cr₄O₄ ⁺. The highest possible total magnetic moments are obtained in suboxides that have Cr-O-Cr bridges with a unique oxygen atom between each pair of Cr atoms. The addition of more oxygen atoms enhances the delocalization of the Cr 3d electrons and reduces the magnetic moment.