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, BnHn (n = 3-5), the lowest energy isomer contains one or more three-membered aromatic BBB rings. Not until B6H6 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(NH2)(n) (n = 4-6). A disagreement of over 20' kcal/mol is found between MP2/6-31G(2d,p)//MP2/631G(d) and B3LYP/6-31G(d)//B3LYP/6-31G(d) for the relative energy of ring and cage structures of B-6(NH2)(6) Calculations on B-4(NH2)(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(NH2)(6) classical structure is a D-3d symmetry chair, while a D-3d cage is predicted to be 15.0 kcal/mol higher in energy.