Xu Lu T, Cooper David L, Dunning Thom H
Department of Chemistry, University of Washington, Seattle, Washington, USA.
Department of Chemistry, University of Liverpool, Liverpool, UK.
J Comput Chem. 2020 Jun 5;41(15):1421-1426. doi: 10.1002/jcc.26185. Epub 2020 Mar 3.
In an earlier study of benzene, Small and Head-Gordon found that the spin-coupled generalized valence bond (SCGVB) wave function for the π system predicted a distorted (non-D ) geometry, one with alternating CC bond lengths. However, the variations in the energy were very small and the predictions were made using a very small basis set (STO-3G). We re-examined this prediction using a much larger basis set (aug-cc-pVTZ) to determine the dependence of the energy of benzene on the distortion angle, Δθ (Δθ = 0° corresponds to the D structure). We also found a distorted geometry with the optimum Δθ being 0.31° with an energy 0.040 kcal mol lower than that for the D structure. In the optimum geometry, adjacent CC bond lengths are 1.3861 Å and 1.4004 Å. Analysis of the SCGVB wave function led us to conclude that the cause of the unusual non-D geometry predicted by the SCGVB calculations seems to be a result of the interaction between the Kekulé and Dewar components of the full SCGVB wave function. The addition of doubly ionic configurations to the SCGVB wave function leads to the prediction of a D geometry for benzene and a dependence on Δθ essentially the same as that predicted by the complete active space self-consistent field wave function.
在早期关于苯的一项研究中，斯莫尔和黑德 - 戈登发现，π 体系的自旋耦合广义价键（SCGVB）波函数预测出一种扭曲的（非 D 型）几何结构，即碳 - 碳键长交替的结构。然而，能量变化非常小，并且这些预测是使用非常小的基组（STO - 3G）做出的。我们使用大得多的基组（aug - cc - pVTZ）重新审视了这一预测，以确定苯的能量对扭曲角 Δθ 的依赖性（Δθ = 0°对应于 D 结构）。我们还发现了一种扭曲的几何结构，其最佳 Δθ 为 0.31°，能量比 D 结构低 0.040 kcal·mol⁻¹。在最佳几何结构中，相邻的碳 - 碳键长分别为 1.3861 Å 和 1.4004 Å。对 SCGVB 波函数的分析使我们得出结论，SCGVB 计算预测的不寻常非 D 型几何结构的原因似乎是完整 SCGVB 波函数的凯库勒和杜瓦成分之间相互作用的结果。向 SCGVB 波函数中添加双离子构型会导致预测出苯的 D 型几何结构，并且对 Δθ 的依赖性与完全活性空间自洽场波函数预测的基本相同。
