Molecular Orbitals of Diborane in Terms of a Gaussian Basis

Abstract

The molecular orbitals of diborane have been calculated by the nonempirical SCF-LCAO method, taking all electrons of the molecule into account and calculating exactly all the integrals. The basis functions were Gaussians centered on the various atoms of the molecule. Two sets of Gaussians have been used, the largest one containing 54 functions, distributed as follows: 9s+3p on each boron, and 3s on each hydrogen. The ordering of the molecular orbitals on the energy scale agrees with that found by Newton, Boer, Palke, and Lipscomb who have used an almost optimized minimal STO basis. The calculated total molecular energy, −52.753 a.u., is 0.468 a.u. higher than the experimental value. A value of 12.9 eV is obtained for the ionization potential, which is to be compared with experimental results ranging from 11.9 to 12.1 eV. An electron-population analysis indicates the presence of very small gross atomic charges (qB=−0.088e; qH(bridge)=−0.002e; qH(terminal)=0.045e). The overlap populations are as follows: n (B, B)=0.064; n (B, Hterm)=0.880; n (B, Hbridge)=0.345. These suggest a structure involving practically no bond between the borons, the bonding in the central part of the system being mainly concentrated in three-center banana bonds.