Department of Chemistry and Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas , Denton, TX 76201, United States.
J Am Chem Soc. 2013 Dec 26;135(51):19282-91. doi: 10.1021/ja409887h. Epub 2013 Dec 13.
Qualitative molecular orbital (MO) theory predicts that square-planar tetrasilacyclobutanetetraone D4h-(SiO)4 should, like D4h-(CO)4, have a triplet ground state, and the results of the (U)CCSD(T)-F12b/cc-pVTZ-F12//(U)B3LYP/6-311+G(2df) calculations, reported here, confirm this expectation. Calculations at the same level of theory find that square-planar tetrasilacyclobutanetetrathione D4h-(SiS)4 also has a triplet ground state. However, these ab initio calculations predict that (SiO)4 and (SiS)4 both have a singlet state of much lower energy, with a tetrahedral (Td) equilibrium geometry and six, electron-deficient, Si-Si bonds. In contrast, the lowest singlet state of (CO)4 and of (CS)4 is calculated to prefer a D4h to a Td geometry. An analysis, based on the second-order Jahn-Teller effect, rationalizes the influence of the electronegativity difference between A and Y in (AY)4 on the energy difference between a D4h and Td geometry. This analysis predicts that (BF)4 and (BCl)4, which are isoelectronic with, respectively, (CO)4 and (CS)4, should both prefer a Td to a D4h equilibrium geometry. These qualitative predictions have been confirmed by our calculations, and (BCl)4 is known experimentally to have a Td equilibrium geometry.
定性分子轨道（MO）理论预测，平面正方形四硅杂环丁烷四酮 D4h-(SiO)4 应该与 D4h-(CO)4 一样，具有三重态基态，这里报道的（U）CCSD(T)-F12b/cc-pVTZ-F12//(U)B3LYP/6-311+G(2df)计算结果证实了这一预期。在相同理论水平的计算中发现，平面正方形四硅杂环丁烷四硫醚 D4h-(SiS)4 也具有三重态基态。然而，这些从头算计算预测（SiO）4 和（SiS）4 都具有能量低得多的单重态，具有四面体形（Td）平衡几何形状和六个电子不足的 Si-Si 键。相比之下，（CO）4 和（CS）4 的最低单重态被计算为优先选择 D4h 而不是 Td 几何形状。基于二阶 Jahn-Teller 效应的分析，合理化了（AY）4 中 A 和 Y 之间的电负性差异对 D4h 和 Td 几何形状之间能量差的影响。该分析预测，分别与（CO）4 和（CS）4 等电子的（BF）4 和（BCl）4 都应优先选择 Td 而不是 D4h 平衡几何形状。这些定性预测已被我们的计算所证实，并且实验上已知（BCl）4 具有 Td 平衡几何形状。
