PNNL

Abstract

The repulsive Coulomb barrier (RCB), an intrinsic potential energy barrier along electron detachment or charge-separation coordinates in multiply charged anions (MCAs), plays a key role in determining MCA electronic and thermodynamic stabilities. Spectroscopic and theoretical characterizations of RCB have been focused on isolated MCAs. In this work, we extend the RCB investigation beyond the previous scope by including noncovalent host-guest cyclodextrin-closo-dodecaborate dianionic complexes ?CD·B12X12 2- (? = a, ß, ?; X = H, F-I). Photodechment photoelectron spectroscopy reveals existence of two distinctly different RCBs, derived from detaching electrons from the guest dianions (RCB1) or ionizing the host neutrals (RCB2), respectively, and the latter being substantially smaller than the former. Theoretical calculations based on the point charge model (PCM) support the duality of RCBs in these complexes. Interestingly, PCM-based quantum energy analyses suggest two tuning mechanisms on RCB1 height. Specifically, the reduced RCB from B12H12 2- to aCD·B12H12 2- upon noncovalently binding with aCD host is driven by Coulombic contributions, while further decreases from aCD·B12F12 2- to aCD·B12I12 2- are primarily achieved by enhanced polarizability attractions. The simulated detachment process reveals positive electrostatic potentials on the remaining monoanion induced by ultrafast outgoing electrons at short ranges, underpinning the dynamic nature of the RCB tuning mechanism.