Electronic structure, isomerism, and chemical bonding in B7- and B7

The electronic structure and chemical bonding of B-7(-) and B-7 have been investigated using photoelectron spectroscopy and ab initio calculations. Complicated, but well-resolved, photoelectron spectra were obtained for B-7(-) at several photon energies and were shown distinctly to contain contributions from different B-7(-) isomers. The structures of the global minima and low-lying isomers were identified using extensive ab initio calculations. Two almost deuenerate pyramidal structures I (C-6upsilon, 3A(1)) and II (C-2upsilon, (1)A(1)) were the most stable for B-7(-). The triplet structure-I is slightly more stable than the singlet structure at out-highest level of theory [RCCSD(T)/6-311+G(2df)] by 0.7 kcal/mol only. The next lowest singlet structure V (C-2upsilon, (1)A(1)) was perfectly planar and was 7.8 kcal/mol higher in energy at RCCSD(T)/6-311+G(2df) level. The observed photoelectron spectra can only be explained when contributions from all these three low-lying isomers were considered; the observed spectral features were in good agreement with the calculated detachment transitions from the three isomers. Analyses of the molecular orbitals and chemical bonding revealed evidence that the triplet pyramidal C-6upsilon structure-I has a twofold (pi and sigma) aroinaticity, the singlet pyrarnidal C-2upsilon structure-II has sigma-aromaticity and pi-antiaromaticity, and the singlet planar C-2upsilon structure V has a twofold (pi and sigma) antiaromaticity.