PNNL

Abstract

Up to now, the smallest known dianions with significant electronic stabilities all contain metal atoms in high oxidation states. Herein, we report an observation of a small, yet remarkably stable metal-free hexacyanodiborate dianion [B2(CN)6]2- that has not been previously observed in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) - a remarkably high value of electronic stability, considering existence of strong electron-electron Coulomb repulsion within the rather small molecular frame on which two extra electrons reside. Photon-energy-dependent experiments and computational investigations reveal a ~2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two B(CN)3- is energetically favorable. Quantum chemical calculations demonstrate a D3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable autophotodetachment via electronically excited dianion resonances.