PNNL

Abstract

Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B6X6]2- (X=Cl-I) are investigated in direct comparison with their closo-[B12X12]2- analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B6X6]2- dianions are electronically significantly less stable than the corresponding [B12X12]2- species. Although [B6Cl6]2- is slightly electronically unstable, [B6Br6]2- and [B6I6]2- are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e- loss) is observed in CID of [B6X6]2- (X=Cl,Br) but not for [B6I6]2-. Halogenide loss (X- loss) is common for [B6X6]2- (X=Br,I) and [B12X12]2- (X=Cl,Br,I). Meanwhile, X• loss is only observed for [B12X12]2- (X=Br,I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e-, X- and X• loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e- and X- losses. A significantly lower RCB for X- loss than for e- loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X- losses are significantly lower for [B6X6]2- than for [B12X12]2-, while enthalpies for X• losses are higher. These observations are consistent with a difference in bond character of the B-X bonds in [B6X6]2- and [B12X12]2-. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B-X bonds in [B12X12]2- have a stronger covalent character than in [B6X6]2-, in which X has a stronger halide character. A part of the computations for this work were done with resources of Leipzig University Computing Centre (M.R. & J.W.). Photoelectron spectroscopy work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences at Pacific Northwest National Laboratory. A portion of this research (E.A.) was performed in Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at PNNL. J.W. is grateful to the Volkswagen foundation for a Freigeist Fellowship.