Ab Initio Study of BnHn and Bn(NH2)n (n = 3−6) Species. A Comparison of Classical and Nonclassical Structures

For early members of the hypercloso boron hydride family, B n H n ( n = 3−5), the lowest energy isomer contains one or more three-membered aromatic BBB rings. Not until B 6 H 6 do cage structures become more stable. When hydrogens are replaced by amino groups, the classical nonplanar ring structure is more stable than the nonclassical cage, B n (NH 2 ) n ( n = 4−6). A disagreement of over 20 kcal/mol is found between MP2/6-31G(2d,p)//MP2/6-31G(d) and B3LYP/6-31G(d)//B3LYP/6-31G(d) for the relative energy of ring and cage structures of B 6 (NH 2 ) 6 . Calculations on B 4 (NH 2 ) 4 including additional electron correlation indicate B3LYP/6-31G(d) is more reliable than MP2/6-31G(2d,p) for relative energies. The lowest energy B 6 (NH 2 ) 6 classical structure is a D 3 d symmetry chair, while a D 3 d cage is predicted to be 15.0 kcal/mol higher in energy.