Web Release Date: March 20,
Electronic Structure, Isomerism, and Chemical Bonding in B7- and B7
Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300
Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99352 and W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, MS K8-88, P.O. Box 999, Richland, Washington 99352
Received: November 3, 2003
In Final Form: February 9, 2004
Abstract:
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 (
and 
) aromaticity, the singlet pyramidal C2v structure-II has -aromaticity
and -antiaromaticity, and the singlet planar C2v structure V has a twofold ( and ) antiaromaticity.
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