Density Functional Theoretical Study on the Electronic Structure of Rh₂O₇ ⁺ with Low Oxidation States

Rh₂O ⁺ (n = 2-10) species are prepared by the reaction of the laser-ablated rhodium atoms with oxygen; furthermore, they are characterized by employing time-of-flight mass spectroscopy. To reveal the stable electronic structure, in this study, we performed the density functional theory calculations for the possible isomers of Rh₂O₇ ⁺. A total of 29 geometries were obtained including cyclic Rh₂O₃, cyclic Rh₂O₂, and ring-opening structures with doublet, quartet, sextet, and octet states. It is noteworthy that no Rh-Rh bond was observed for all the optimized Rh₂O₇ ⁺ isomers including oxides, peroxides, superoxides, and oxygen groups. The optimized geometries were also confirmed to exhibit minimum structural energies by employing harmonic frequency analysis at the same energy level. Generally, two types of oxygen-bridged geometries were discovered with cyclic and pseudo-linear Rh₂O₇ ⁺, which contained one or more than one O₂ groups. It is concluded that the cyclic structure comprises a lower energy than that observed in pseudo-linear structures. In addition, Rh₂O₇ ⁺ tends to be unstable when the coordination groups change from O₂ to O₂ ^- unit. Finally, the localized orbital bonding analysis indicates that Rh has oxidation states of 1 or 2 in cyclic Rh₂O₇ ⁺ structures; this is true even in the presence of O^2-, O₂ ^-, and O₂ ^2- groups.