Further Study of Umbrella vs Bridged Geometries: SCF–MO and CI Calculations for C2H6++ and Ammonia Borane

The potential surfaces and molecular properties of ammonia borane BNH₆ and certain positive ions of ethane are investigated by means of SCF–MO and CI calculations for these systems. The role a single molecular orbital plays in determining whether the equilibrium configuration of A₂H₆ and related molecules possess umbrella or bridged arrangements of their hydrogen atoms is re‐emphasized by the work. The calculations also indicate that differences in the effectiveness of boron and carbon $p$ AO's restrict the class of molecules with bound states in umbrella conformations to those containing carbon (or nitrogen) atoms; at the same time it appears that only boron compounds can have bound states for bridged hydrogen arrangements. Even though BHN₆ has less symmetry than A₂H₆ systems its geometry is also found to be easily interpretable in terms of the behavior of one of its MO's, the ${\text{2b}}_1\text{→2e}$ , and because of the basic similarity between the latter and its C₂H₆ counterpart, the molecule as a whole is also observed to favor an umbrella geometry; further comparison finds ammonia borane to possess a smaller total binding energy, major force constant, and minimum ionization potential than does ethane. Charge‐density contour diagrams clearly show that BNH₆ is characterized by a weak $\mathrm{p\sigma }$ dative bond, originating from an $\mathrm{N\to B}$ charge transfer which, in turn, produces a rather large experimental dipole moment of 4.9 D; the calculated result from the best wavefunction obtained (total energy estimated to be 0.1 hartree above Hartree–Fock limit) is 5.71 D. Finally a large CI calculation is found to produce only a small effect on the single determinantal representation of the ground state; only far‐uv electronic transitions are found, with the lowest singlet–singlet ${(}^1{\mathrm{E\leftarrow }}^1{A}_1)$ occurring at 74 000 cm⁻¹.