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引用次数: 0
摘要
众所周知,环戊二烯阴离子的自旋耦合(SC)描述存在问题。对该阴离子的六轨道六电子自旋耦合计算打破了 D5h 电子密度空间对称性,将六个 pi 单占轨道中的两个近似定位在一个碳上。对同样的六个轨道上的六个 pi 电子进行的完全活性空间价键(CASVB)计算并没有打破对称性,在五个碳上产生了五个等效的 pi 轨道和一个沿 C5 分子旋转轴具有高振幅的 pi 轨道。自旋耦合计算包括五个轨道中的六个电子,即 SC(6,5),在碳上产生五个等效的非正交 pi 轨道,每个轨道的电子占有率为 "1.2"(6/5)。在本文中,这些不同的解法与本文作者提出的解法进行了对比。在作者提出的解法中,完全配对对称破缺解法的五个空间不同但等价的构型被组合在一起,进行了 30 个轨道中六个电子的多构型自旋耦合计算。结论是,尽管这种计算很复杂,但其结果在性质上更类似于对环戊二烯阴离子共振 pi 系统的价键式描述。
On the Spin-Coupled description of the pi system of the cyclopentadienyl anion
It is well-known that the Spin-Coupled (SC) description of the cyclopentadienyl anion is problematic. A converged six electrons in six orbitals SC calculation on this anion break the D5h electron density spatial symmetry by approximately localizing two of the six pi singly-occupied orbitals over one carbon. A complete active space valence bond (CASVB) calculation for the same six pi electrons in six orbitals does not break symmetry, yielding five equivalent pi orbitals over the five carbons and one pi orbital with high amplitude along the C5 molecular rotation axis. A Spin-Coupled calculation comprising six electrons in five orbitals, SC(6,5), yields five equivalent non-orthogonal pi orbitals over the carbons with “1.2” (6/5) electron occupancy each. In this paper, these different solutions are contrasted with one generated by the present author in which five spatially different but equivalent configurations of a perfect-pairing symmetry broken solution are put together in a multi-configuration Spin-Coupled calculation of six electrons in 30 orbitals. It is concluded that, in spite of the computational complexity of this calculation, its result is qualitatively more akin to a valence bond-like description of the resonant pi system of the cyclopentadienyl anion.
期刊介绍:
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