New insights into dianion-cation contact ion-pairs: understanding the effect of cation complexation on the electron detachment and ionic fragmentation pathways of multiply charged anions

Contact ion-pairs play crucial roles in chemical systems, but remain relatively uncharacterized as isolated entities in the gas-phase. We highlight recent findings from our laboratory on one class of ion-pairs, namely dianion-cation complexes. Following a brief review of the properties of isolated multiply charged anions (MCAs), we present experimental ( low energy collisional excitation measurements) and computational studies ( density functional theory) to explore how the intrinsic properties of an MCA are perturbed upon complexation with a cation. Excitation of both the bare Pd(CN)(4)(2-) dianion and the K+ center dot Pd(CN)(4)(2-) ion-pair results in fragmentation with production of the Pd(CN)(3)(-) product ion, consistent with decay of the dianion via ionic fragmentation. This illustrates that the ground-state ion-pair potential energy surface is dominated by the intrinsic characterisitics of the MCA, providing direct evidence that the cation only weakly perturbs the electronic structure of an MCA, consistent with a contact ion-pair. Density functional theory calculations of the fragmentation potential energy surface support the experimental results, and reveal the existence of a novel double-minima surface, separated by a repulsive-Coulomb barrier-like (RCB-like) feature at short range. We extend these calculations to explore whether similar RCB-like features also exist on the dianion-cation electron detachment surfaces. One-dimensional cuts through the electron detachment potential energy surface of the K+ center dot PtCl62- complex were calculated using a point charge model. The calculations reveals that electron detachment along a direction distant from the K+ cation results in an RCB-like feature on the potential energy surface. The experimental implications of such features are discussed.