Theoretical study of hydrogenation of the doubly aromatic B cluster

We have studied the influence of hydrogenation on the relative stability of the low-lying isomers of the anionic B cluster, computationally. It is known that the pure-boron B cluster has a doubly (σ- and π-) aromatic C (A) quasi-planar wheel-type triplet global minimum (structure 1), a low-lying σ-aromatic and π-antiaromatic quasi-planar singlet C ( A) isomer 2 (0.7 kcal mol above the global minimum), and a planar doubly (σ- and π-) antiaromatic C (A) isomer 3 (7.8 kcal mol above the global minimum). However, upon hydrogenation, an inversion in the stability of the species occurs. The planar BH (C, A) isomer 4, originated from the addition of two hydrogen atoms to the doubly antiaromatic B isomer 3, becomes the global minimum structure. The second most stable BH isomer 5, originated from the quasi-planar triplet wheel isomer 1 of B, was found to be 27 kcal mol 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, A) of B upon H -addition. In contrast, the planar isomer of B (C, A) gains aromatic character upon addition of two hydrogen atoms, which makes it more stable. © Springer-Verlag 2005.