Electronic Structure, Isomerism, and Chemical Bonding in B7-and B7

The electronic structure and chemical bonding of B7- and B7 have been investigated using photoelectron spectroscopy and ab initio calculations. Complicated, but well-resolved, photoelectron spectra were obtained for B7- at several photon energies and were shown distinctly to contain contributions from different B7- isomers. The structures of the global minima and low-lying isomers were identified using extensive ab initio calculations. Two almost degenerate pyramidal structures I (C6v, 3A1) and II (C2v, 1A1) were the most stable for B7-. The triplet structure-I is slightly more stable than the singlet structure at our highest level of theory RCCSD(T)/6-311+G(2df) by 0.7 kcal/mol only. The next lowest singlet structure V (C2v, 1A1) 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 C6v structure-I has a twofold (π and σ) aromaticity, the singlet pyramidal C2v structure-II has σ-aromaticity and π-antiaromaticity, and the singlet planar C2v structure V has a twofold (π and σ) antiaromaticity.