科研之友
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摘要
At the B3LYP/6-311 G** and the BP86/6-311 G** levels of theory, BNN, H3BNN, NNBH2-BH2NN, (BNN)(2)ZH(2), NNBBNN, (BNN)(3)( ), (BNN)(4), (BNN)(4)(2 ), (BNN)(4)(2-), B-4(NN)(2). (BNN)(5)(-), (BNN)(6), (BNN)(7)( )(,) and (BNN)(8)(2 ) are investigated. Neutral (BNN)(4) is aromatic with its triplet state but antiaromatic with its singlet state. (BNN)4 dication favors D-2d structure, while (BNN)(4) dianion favors a planar D-4h structure. (BNN)(3)( ), (BNN)(4)(2-), (BNN)(5)(-), (BNN)(6), (BNN)(7)( ), (BNN)(7)(3-), (BNN)(8)(2 ), and (BNN)(8)(2-) are all aromatic with planar monocyclic conformation, following the 4n 2 rule. Moreover, according to the CASSCF and MRCI calculations, the planar B-4(NN)(2) of D-2h symmetry prefers to be a sigma-pi diradical in spite of open-shell singlet or triplet and is also aromatic. Akin to the sigma-pi back interaction in compounds containing transition metal, there exists the sigma-pi back interaction between boron and N-2 ligand among some species reported herein, which strengthens B-N bond but activates N-N bond, especially in (4)Sigma(-) BNN. The T-shaped structure lies lowest in energy among seven isomers of the (BNN)(2)H-2 dimer, and the parallel-displaced structure is favored between two isomers of the (BNN)(6) dimer.
