缺失的理解共轭理论的关键一环 - As easy as π

莫亦荣教授（美国西密西根大学教授，厦门大学客座教授）与吴玮教授（厦门大学）团队提出并验证了一个新的见解，分子内多键张力，即π–π排斥力，是解释共轭会破坏某些分子稳定性的关键。他们发现π电子排斥力比人们之前认为的要重要得多。这一发现成功地弥合了共振能实验数值和理论计算数值之间的偏差，有可能改变人们对共轭的理解。

“共轭的重要性被过分强调，而同等重要的分子间多键张力却没有受到应有的重视。现在是时候改写我们的教科书了”，计算化学领域专家，耶路撒冷希伯来大学的Sason Shaik如此评论道。美国密苏里大学电子结构理论领域的专家Rainer Glaser对此表示赞同，“这是令人信服的讨论，应该而且将会被包含在教科书中。”

下面是Chemsitry World关于这篇文章的详细报道：

Scientists’ discovery that π electron repulsion is more important than previously thought might change our understanding of conjugation. The effect is behind peculiar irregularities in bond lengths and resonance energies of conjugated molecules that our textbooks cannot explain.

Source: © Royal Society of Chemistry

Repulsion between p electrons can explain apparently counterintuitive destabilising effects found in some conjugated molecules

Conjugated molecules like 1,3-pentadiene have p electrons that are delocalised across several bonds. Conjugation lowers a molecule’s overall energy and makes it more stable. However, a few years ago, researchers from Long Island University, US, claimed that conjugation could actually destabilise some molecules. They calculated cyanogen (NC–CN) to be less stable than its non-conjugated analogue ethylenediamine (H₂NCH₂–CH₂NH₂). Many researchers have tried to disprove this, but so far, no one has been able to give a simple explanation for these apparently counterintuitive results.

Now, Yirong Mo from Western Michigan University, US, and colleagues discovered the missing factor to explain this strange destabilising effect: repulsion between π electrons. ‘I realised that all sides missed one key, the π–π repulsion,’ Mo says. Using a non-conjugated and a conjugated model system B₂H₄ and B₄H₂) Mo and his team computationally measured repulsion between different types of bonds. They found that repulsion between conjugated p bonds was stronger than previously assumed – stronger even than the repulsion between adjacent bonds, which pushes the molecule into the lowest energy conformation.

‘There has been overemphasis on conjugation, while the intramolecular multi-bond strain, which is at least equally important, has not received much attention. It’s about time to change the textbooks,’ comments Sason Shaik, an expert in computational chemistry at the Hebrew University, Jerusalem. Rainer Glaser, an expert in electronic structure theory at the University of Missouri, US, agrees. ‘This discussion is compelling and it should be and will have to be included in textbooks’.

Mo’s work can explain previously mysterious discrepancies between experimental and theoretical measurements, such as the longer carbon–nitrogen bond in nitrobenzene than in aniline; nitrobenzene’s aromatic ring and conjugated NO₂ group repel each other through p–p repulsion, which doesn’t exist in aniline’s non-conjugated NH₂ group.

Ángel Martín Pendás, a chemical bonding theory expert at the University of Oviedo, Spain, finds ‘the multi-bond strain concept a truly interesting proposal that deserves deeper scrutiny, and that will provide chemists with many hours of fun’.

Read abstract of this article:

Intramolecular multi-bond strain: the unrecognized side of the dichotomy of conjugated systems

Yirong Mo,* Huaiyu Zhang, Peifeng Su, Peter D. Jarowski* and Wei Wu*  

Chem. Sci., 2016, 7, 5872-5878

DOI: 10.1039/C6SC00454G

Electron conjugation stabilizes unsaturated systems and diminishes the differences among bond distances. Experimentally, Kistiakowsky and coworkers first measured and noticed the difference between the hydrogenation heats of carbon–carbon double bonds in conjugated systems. For instance, the hydrogenation heat of butadiene is 57.1 kcal mol−1, which is less than two times that of the hydrogenation heat of 1-butene (30.3 kcal mol−1), and the difference (3.5 kcal mol−1) is the extra stabilization due to the resonance between two double bonds in the former, and is referred to as the experimental resonance energy. Following Kistiakowsky's definition, Rogers et al.studied the stepwise hydrogenation of 1,3-butadiyne and concluded that there is no conjugation stabilization in this molecule. This claim received objections instantly, but Rogers and coworkers further showed the destabilizing conjugation in 2,3-butanedione and cyanogen. Within resonance theory, the conjugation energy is derived “by subtracting the actual energy of the molecule in question from that of the most stable contributing structure.” The notable difference between the experimental and theoretical resonance energies lies in that the former needs other real reference molecules while the latter does not. Here we propose and validate a new concept, intramolecular multi-bond strain, which refers to the repulsion among π bonds. The π–π repulsion, which is contributed to by both Pauli exchange and electrostatic interaction, is quantified with the B4H2model system (16.9 kcal mol−1), and is compared with the σ–σ repulsion in B2H4 (7.7 kcal mol−1). The significance of the π–π repulsion can be demonstrated by the much longer carbon–nitrogen bond in nitrobenzene (1.486 Å) than in aniline (1.407 Å), the very long and weak nitrogen–nitrogen bond (1.756 Å) in dinitrogen tetroxide, and the instability of long polyynes. This new concept successfully reconciles the discrepancy between experimental and theoretical conjugation energies. However, we maintain that by definition, electron conjugation must be stabilizing.

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