A Computational Determination of the Lowest Energy Electronic and Geometric States of First Row Transition Metal Dioxygen Dications

The lowest energy geometric structures and electronic spin states of first row transition metal (TM) dioxygen dication molecules ([TM–O2]2+; TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) have been determined at the B3LYP/LANL2DZ level of theory (along with an extra d -type polarization function added to the O atoms). In order to further verify the spin states, CASSCF(6 + n , 9) energy points were determined ( n = number of TM d electrons). It has been found that with the exception of [Sc–O2]2+, [V–O2]2+, [Co–O2]2+, and [Ni–O2]2+, all [TM–O2]2+ molecules take on a high-spin state. [Sc–O2]2+ adopts a trigonal structure, while [Ti–O2]2+-[Mn–O2]2+ are essentially linear and [Fe–O2]2+-[Zn–O2]2+ are bent. It is further noted that the O–O bond decreases from 130.0 pm to 118.1 pm as the TM changes from Sc to Zn. However, the TM–O2 bond lengths fluctuate between values of 182.2 pm for [Ni–O2]2+ and 232.2 pm for [Zn–O2]2+.