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 BNH 6 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 A 2H6 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 A 2H6 systems its geometry is also found to be easily interpretable in terms of the behavior of one of its MO's, the 2b 1→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 pσ dative bond, originating from an N→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 (1E←1A1) occurring at 74 000 cm-1.