Structure and stability of closo-b-{n}h-{n-1}n-{2}^{-} (n=5-12)

$Closo-B-{n}H-{n-1}N-{2}^{-} (n=5-12)$, framework- isoelectronic to $closo-B-{n}H-{n}^{2-}, have been investigated at the B3LYP/6-311+G** level of theory. The most stable positional isomers of individual $closo-B-{n}H-{n-1}N-{2}^{-} are similar to those of $closo-CB-{n-1}H-{n}^{-} and $closo-B-{n}H-{n-1}CO^{-} in framework configuration. The energy analysis reveals that the cage $B-{12}H-{11}N-{2}^{-} with the icosahedron framework, $B-{6}H-{5}N-{2}^{-} with the octahedron framework and $B-{10}H-{9}N-{2}^{-} with the bicapped square antiprism framework are, in order, the most stable in the $B-{n}H-{n-1}N-{2}^{-} (n=5-12)$ system. Furthermore, three-dimensional aromaticity of all $closo-B-{n}H- {n-1}N-{2}^{-}$ is characterized. The most stable cages, $B-{12}H-{11}N-{2}^{-}, $B-{10}H-{9}N-{2}^{-} and $B-{6}H-{5}N-{2}^{-}, also have relatively large aromaticity. All the predicted N-N stretching frequencies (2127-2303 cm^-1) of the most stable positional isomers in the $B-{n}H-{n-1}N-{2}^{-} system are lower than the computational N-N stretching frequency (2445 cm^-1) of free N₂ at the same level, which results from the σ donation of N₂ to B frameworks. The stretching frequencies of N-N in the $B-{n}H-{n-1}N-{2}^{-} cages increase with the decrease of N-N distances or the increase of B-N distance. In addition, B_n-1H_n-1 fragment has larger affinity for group CH than that for group BNN, namely, it favors to form the $closo-CB-{n-1}H-{n}^{-} rather than the $closo-B-{n}H-{n-1}N-{2}^{-}.