Theoretical study of hydrogenation of the doubly aromatic B-7

We have studied the influence of hydrogenation on the relative stability of the low-lying isomers of the anionic B-7(-) cluster, computationally. It is known that the pure-boron B-7(-) cluster has a doubly (sigma- and pi-) aromatic C-6v ((3)A(1)) quasi-planar wheel-type triplet global minimum (structure 1), a low-lying sigma-aromatic and pi-antiaromatic quasi-planar singlet C-2v ((1)A(1)) isomer 2 (0.7 kcal mol(-1) above the global minimum), and a planar doubly (sigma- and pi-) antiaromatic C-2v ((1)A(1)) 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 B7H2- (C-2v, (1)A(1)) isomer 4, originated from the addition of two hydrogen atoms to the doubly antiaromatic B-7(-) isomer 3, becomes the global minimum structure. The second most stable B7H2- isomer 5, originated from the quasi-planar triplet wheel isomer 1 of B-7(-), 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 (C-6v, (3)A(1)) of B-7(-) upon H-2- addition. In contrast, the planar isomer of B-7(-) (C-2v, (1)A(1)) gains aromatic character upon addition of two hydrogen atoms, which makes it more stable.