Deviations from Born-Oppenheimer theory in structural chemistry: Jahn-Teller, pseudo Jahn-Teller, and hidden pseudo Jahn-Teller effects in C3H3 and C3H3(-).
Deviations from Born-Oppenheimer theory in structural chemistry: Jahn-Teller, pseudo Jahn-Teller, and hidden pseudo Jahn-Teller effects in C3H3 and C3H3(-).
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DOI：
10.1021/jp403034c
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发表时间：
2013-08
期刊：
影响因子：
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通讯作者：
H. Kayı;P. García-Fernández;I. Bersuker;J. E. Boggs
中科院分区：
文献类型：
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作者：
H. Kayı;P. García-Fernández;I. Bersuker;J. E. Boggs
详细研究了自由基C3 H3和阴离子C3 H3（-）两个相似分子体系基态和激发态的电子结构和电子振动耦合，揭示了它们的平衡结构是如何偏离Born-Oppenheimer近似，起源于至少两个电子态的电子振动混合，产生Jahn-Teller（JT）、赝JT（PJT）和隐藏的PJT效应.从C_3H_3的高对称性几何构型D_3h出发，计算了其二重简并基态（2）E″和两个最低激发态（2）A_1 ′和（2）E ′，发现它们都通过JT电子振动耦合问题E″ E ′和两个PJT问题对基态的畸变有贡献（E″ + A1 '）e″和（E″ + E'）（a2″ + e″）;所有三个有效简正模e '（1335 cm（-1））、e″（1030 cm（-1））和a2″（778 cm（-1））都是虚模，这意味着所有三个振动耦合都足够强，导致不稳定性，尽管方向不同。第一种是基态JT效应，它增强了其中一个C-C键（形成C2 v对称的烯键式），而两种PJT效应分别产生氢原子的顺式（a2″朝向C3 v对称）和反式（e″）褶皱。因此，C3 H3具有两种共存的具有不同几何构型的平衡构型。在C3 H3（-）阴离子中，D3 h对称的基态电子态是轨道非简并自旋三重态（3）A2'，具有一组能量接近的单重态和三重态激发态，顺序为（1）A1'，（3）A1“，（1）E "，（3）E”和（1）E '。这表明（3）A2' +（3）A1″）a2″和（3）A2' +（3）E″）e″两种PJT耦合可能影响（3）A2'态平衡结构的几何形状。事实上，在这种状态下，两个振动模式a2″（1034 cm（-1））和e″（1284 cm（-1））都是虚的。类似于自由基的情况，它们分别产生氢原子的顺式（a2″）和反式（e″）褶皱，但基本C3三角形没有e'畸变;具有Cs对称性的平衡构型沿着较强的e″畸变发生。另一个具有C2 v对称性的更高能量的三重态最小值是激发态（3）E″电子态中强JTE的结果。除了这些APES最小值与自旋三重态电子态，该系统有一个共存的最小值与自旋单重态电子态，这是由于隐藏的PJT效应耦合两个单重态激发态。具有不同几何形状和自旋的C3 H3（-）阴离子的两种最低平衡构型实现了（所有电子e（2）构型所共有的）磁性和结构双稳态，并伴有自旋交叉。一些一般光谱的后果也注意到。作为一个整体，这篇文章的目的是证明效率的振动耦合方法在合理化的起源复杂的结构特征的分子系统，由于非绝热JT效应的组合。
The electronic structure and vibronic coupling in two similar molecular systems, radical C3H3 and anion C3H3(-), in ground and excited states, are investigated in detail to show how their equilibrium structures, in deviation from the Born-Oppenheimer approximation, originate from the vibronic mixing of at least two electronic states, producing the Jahn-Teller (JT), pseudo JT (PJT), and hidden PJT effects. Starting with the high-symmetry geometry D3h of C3H3, we evaluated its 2-fold degenerate ground electronic state (2)E″ and two lowest excited states (2)A1' and (2)E' and found that all of them contribute to the distortion of the ground state via the JT vibronic coupling problem E″ ⊗ e' and two PJT problems (E″ + A1') ⊗ e″ and (E″ + E') ⊗ (a2″ + e″); all the three active normal modes e'(1335 cm(-1)), e″(1030 cm(-1)), and a2″(778 cm(-1)) are imaginary, meaning that all the three vibronic couplings are sufficiently strong to cause instability, albeit in different directions. The first of them, the ground state JT effect, enhances one of the C-C bonds (toward an ethylenic form with C2v symmetry), while the two PJT effects produce, respectively, cis (a2″ toward C3v symmetry) and trans (e″) puckering of the hydrogen atoms. As a result, C3H3 has two coexisting equilibrium configurations with different geometry. In the C3H3(-) anion, the ground electronic state in D3h symmetry is an orbitally nondegenerate spin triplet (3)A2' with a group of close in energy singlet and triplet excited states in the order of (1)A1', (3)A1″, (1)E″, (3)E″, and (1)E'. This shows that two PJT couplings, ((3)A2' + (3)A1″) ⊗ a2″ and ((3)A2' + (3)E″) ⊗ e″, may influence the geometry of the equilibrium structure in the (3)A2' state. Indeed, both vibrational modes, a2″(1034 cm(-1)) and e″(1284 cm(-1)), are imaginary in this state. Similar to the radical case, they produce, respectively, cis (a2″) and trans (e″) puckering of the hydrogen atoms, but no e' distortion of the basic C3 triangle; the equilibrium configuration with Cs symmetry occurs along the stronger e″ distortions. Another higher-in-energy triplet-state minimum with C2v symmetry emerges as a result of a strong JTE in the excited (3)E″ electronic state. In addition to these APES minima with spin-triplet electronic states, the system has a coexisting minimum with a spin-singlet electronic state, which is shown to be due to the hidden PJT effect that couples two singlet excited states. The two lowest equilibrium configurations of the C3H3(-) anion with different geometry and spin realize a (common to all electronic e(2) configurations) magnetic and structural bistability accompanied by a spin crossover. Some general spectroscopic consequences are also noted. As a whole, this article is intended to demonstrate the efficiency of the vibronic coupling approach in rationalizing the origin of complicated structural features of molecular systems as due to a combination of nonadiabatic JT effects.