Ab initio study of the hypercloso boron hydrides BnHn and BnHn-.: Exceptional stability of neutral B13H13

The neutral hypercloso boron hydrides BnHn (n = 5-13, 16, 19, 22) and the boron hydride radical monanion BnHn- (n = 5-13) have been studied at the B3LYP/6-31G* and higher levels of density functional theory. The pairing and capping principles help rationalize the lowest energy structures. Neutral boron hydrides BnHn with n = 3p + 1 (p = 2-7) all have one boron atom on a 3-fold axis. Loss of one electron from BnHn2- (n = 5-13) is predicted to be exothermic except for B12H122- This, the most stable and aromatic dianion, has a 10.9 kcal/mol (B3LYP/6-311+G**//B3LYP/6-31G*) adiabatic ionization potential. The relative stabilities of the neutral and monanion boron hydrides were evaluated using the equation B2H2x + (n - 2)BHinc --> BnHnx, in which x = 0 and -1 and BHinc is the energy difference between B3H5 (C-2 nu, planar) and B2H4 (D-2h) Removing two electrons from aromatic dianions changes the nucleus-independent chemical shifts (NICS) from negative (aromatic) to positive (antiaromatic) except for neutral B6H6, B7H7, B12H12 (C-2 nu), and B13H13 (C-3 nu, C-2 nu, C-s). The NICS values (ca. -21.0 ppm) for the three neutral B13H13 forms are comparable with the B13H13-dianion value (-23.3 ppm). Interestingly, for all these species, the NICS values at points 1.0 Angstrom, above the centers of the unique triangles are all negative, which is in contrast to the case for benzene with well-known shielding and deshielding regions. On the basis of the stabilization energy and other aromatic criteria, the neutral B13H13 boron hydride appears to be a good candidate for synthesis.