Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, USA.
J Phys Chem A. 2011 Jun 2;115(21):5478-87. doi: 10.1021/jp202671u. Epub 2011 May 10.
In light of the important recent synthesis of a stable tetrasilacyclobutadiene dianion compound by Sekiguchi and co-workers and the absence of theoretical studies, ab initio methods have been used to investigate this dianion and a number of related species. These theoretical methods predict multiple minima for each compound, and most minima contain folded and bicyclic silicon rings. For (Si(4)H(4))(2-), (Si(4)H(4))(2-)·2Li(+), Si(4)(SiH(3))(4)·2Li(+), Si(4)(SiH(3))(4)·2Na(+), and Si(4)(SiH(3))(4)·2K(+), respectively, the energetically lowest-lying structures are designated A-3 (C(2v) symmetry), B-8 (C(1) symmetry), C-1 (C(2) symmetry), D-1 (C(2) symmetry), and E-1 (C(2h) symmetry). None of these structures satisfies both the ring planarity and the cyclic bond equalization criteria of aromaticity. However, all of the representative NICS values of these lowest-lying structures are negative, indicating some aromatic character. Especially, structures C-1 and D-1 of C(2) symmetry effectively satisfy the criteria of aromaticity due to the slightly trapezoidal silicon rings, which are nearly planar with nearly equal bond lengths. SiH(3) substitution for hydrogen in (Si(4)H(4))(2-)·2Li(+) significantly reduces the degree of aromaticity, as reflected in the substantially smaller NICS absolute values for Si(4)(SiH(3))(4)·2Li(+) than those of (Si(4)H(4))(2-) and (Si(4)H(4))(2-)·2Li(+). The aromaticity is further weakened in Si(4)(SiH(3))(4)·2Na(+) and Si(4)(SiH(3))(4)·2K(+) by replacing lithium with the sodium and potassium cations.
鉴于 Sekiguchi 及其同事最近对稳定的四硅环丁二烯二阴离子化合物的重要综合，以及缺乏理论研究，我们使用从头算方法研究了这种二阴离子和一些相关的物种。这些理论方法预测每个化合物都有多个最小值，并且大多数最小值都包含折叠和双环硅环。对于 (Si(4)H(4))(2-)、(Si(4)H(4))(2-)·2Li(+)、Si(4)(SiH(3))(4)·2Li(+)、Si(4)(SiH(3))(4)·2Na(+) 和 Si(4)(SiH(3))(4)·2K(+)，分别为，能量最低的结构指定为 A-3（C(2v) 对称性）、B-8（C(1) 对称性）、C-1（C(2) 对称性）、D-1（C(2) 对称性）和 E-1（C(2h) 对称性）。这些结构中没有一个满足芳香性的环平面和环状键等化标准。然而，这些最低能量结构的所有代表性 NICS 值均为负值，表明具有一定的芳香性。特别是 C(2) 对称性的 C-1 和 D-1 结构由于硅环略微梯形，几乎是平面的，键长几乎相等，因此有效地满足了芳香性的标准。在 (Si(4)H(4))(2-)·2Li(+)中的氢被 SiH(3)取代会显著降低芳香性，这反映在 Si(4)(SiH(3))(4)·2Li(+) 的 NICS 绝对值比 (Si(4)H(4))(2-) 和 (Si(4)H(4))(2-)·2Li(+)小得多。通过用钠离子和钾离子代替锂离子，Si(4)(SiH(3))(4)·2Na(+) 和 Si(4)(SiH(3))(4)·2K(+) 中的芳香性进一步减弱。
