Photoelectron spectroscopy and ab initio study of the doubly antiaromatic B_6~(2-) dianion in the LiB_6~- cluster

摘要

A metal-boron mixed cluster LiB_6~- was produced and characterized by photoelectron spectroscopy and ab initio calculations.A number of electronic transitions were observed and used to compare with theoretical calculations.An extensive search for the global minimum of LiB_6~- was carried out via an ab initio genetic algorithm technique.The pyramidal C_(2v) (~1A _1) molecule was found to be the most stable at all levels of theory.The nearest low-lying isomer was found to be a triplet C_2 (~3B) structure,9.2 kcal/mol higher in energy.Comparison of calculated detachment transitions from LiB_6~- and the experimental photoelectron spectra confirmed the C_(2v) pyramidal global minimum structure.Natural population calculation revealed that LiB_6~- is a charge-transfer complex,Li~+B_6~(2-) ,in which Li~+ and B_6~(2-) interact in a primarily ionic manner.Analyses of the molecular orbitals and chemical bonding of B_6~(2-) showed that the planar cluster is twofold (pi- and sigma-) antiaromatic,which can be viewed as the fusion of two aromatic B_3~- units.