Electronic Instability and Solvation Stabilization of Oxocarbon Dianions (CO)^2- (n = 4-6)

Oxocarbon dianions (CO)^2- have been recently found to be promising candidates in the design of high-capacity and fast rechargeable batteries but are intrinsically unstable in the isolated form. Fundamental understandings of their electronic structures, solvent stabilization, and interactions with solvents and counterions are crucial in comprehending their electron transfer reactions occurring in batteries. In this article, we employed microsolvated dianionic clusters as models and combined negative ion photoelectron spectroscopy (NIPES) and theoretical computations to probe the electronic instability and solvation stabilization of (CO)^2- (n = 4-6) dianions. Through the smallest observable members in each series of microhydrated dianions and their recorded adiabatic and vertical detachment energies (ADEs and VDEs), the minimum numbers of H₂O molecules required to stabilize (CO)^2- dianions are determined to be 4, 3, and 2 for n = 4, 5, and 6, respectively, while 3 and 2 water molecules can make (C₄O₄)^2- and (C₅O₅)^2- metastable and detectable. Using theoretical calculations, we determined the lowest energy structures of each complex. The first few H₂O molecules prefer to be directly hydrogen bonded to two adjacent O atoms around the oxocarbon ring. The water binding strengths are generally comparable when each H₂O molecule is bound at a separate binding pocket, but the binding strengths decrease when all binding pockets are occupied, in parallel with the observed ADE and VDE shift trends. Moreover, hydrated (C₄O₄)^2- dianions are found to possess distinct electronic structures compared to its (C₅O₅)^2- and (C₆O₆)^2- analogues due to its near-degenerate HOMO and HOMO-1, while there exists a larger gap for the latter two dianions. Upon hydration, the overall electronic structure patterns are maintained without much distortion, but fine changes are noticeable, which warrant future studies.