Theoretical study of hydrogenation of the doubly aromatic B7- cluster.

We have studied the influence of hydrogenation on the relative stability of the low-lying isomers of the anionic B7- cluster, computationally. It is known that the pure-boron B7- cluster has a doubly (sigma- and pi-) aromatic C6v(3A1) quasi-planar wheel-type triplet global minimum (structure 1), a low-lying sigma-aromatic and pi-antiaromatic quasi-planar singlet C2v (1A1) isomer 2 (0.7 kcal mol(-1) above the global minimum), and a planar doubly (sigma- and pi-) antiaromatic C2v (1A1) isomer 3 (7.8 kcal mol(-1) above the global minimum). However, upon hydrogenation, an inversion in the stability of the species occurs. The planar B7H 2- (C2v, 1A1) isomer 4, originated from the addition of two hydrogen atoms to the doubly antiaromatic B7- isomer 3, becomes the global minimum structure. The second most stable B7H2- isomer 5, originated from the quasi-planar triplet wheel isomer 1 of B7- , was found to be 27 kcal mol(-1) higher in energy. The inversion in stability occurs due to the loss of the doubly aromatic character in the wheel-type global minimum isomer (C6v, 3A1) of B7- upon H2-addition. In contrast, the planar isomer of B7- (C2v, 1A1) gains aromatic character upon addition of two hydrogen atoms, which makes it more stable.