Danovich David, Foroutan-Nejad Cina, Hiberty Philippe C, Shaik Sason
Institute of Chemistry, Hebrew University of Jerusalem , 9190401 Jerusalem, Israel.
CEITEC - Central European Institute of Technology, Masaryk University , Kamenice 5/A4, CZ-62500 Brno, Czech Republic.
J Phys Chem A. 2018 Feb 22;122(7):1873-1885. doi: 10.1021/acs.jpca.7b11919. Epub 2018 Feb 8.
We analyze the properties of 15 3-electron bonds, which include σ-3-electron-bonds, such as dihalide radical anions and di-noble gas radical cations, π-3-electron-bonds as in hydrazine radical cations, and doubly-π-(3e)-bonded species such as O, FeO, S, etc. The primary analytical tool is the breathing-orbital valence-bond (BOVB) method, which enables us to quantify the charge shift resonance energy (RE) of the three electrons, and the bond dissociation energies (D). BOVB is tested reliable against MRCI calculations. Our findings show that in all 3-electron bonds, none of the VB structures have by themselves any bonding. In fact, in each VB structure, the three electrons maintain Pauli repulsion, while the entire bonding energy arises from resonance due to the charge shift between the two or more constituent VB structures. Hence, 3e-bonds are charge shift bonds (CSBs). The CSB character is probed by calculating the Laplacian (L) of the 3e-bond. Thus, much like the CSBs in electron-pair bonds, such as F or the central bond in [1.1.1]propellane, here too L is positive, thus showing the excess kinetic energy of the shared density due to the Pauli repulsion in the 3-electron VB structures. The RE values for 3-electron bonds are invariably larger than the corresponding bond energies. For the doubly-π-(3e)-bonded species, RE is very large, exceeding 100 kcal mol. As such, it is fitting to conclude that σ- and π-3-electron-bonds find their natural place in the CSB family along with two-electron CSBs, with which they share identical energetic and topological characteristics. Experimental manifestations/tests of 3e-CSBs are proposed.
我们分析了15种三电子键的性质，其中包括σ-三电子键，如二卤化物自由基阴离子和二稀有气体自由基阳离子；π-三电子键，如肼自由基阳离子中的键；以及双π-(3e)键合物种，如O、FeO、S等。主要的分析工具是呼吸轨道价键（BOVB）方法，它使我们能够量化三个电子的电荷转移共振能（RE）和键解离能（D）。BOVB方法经多参考组态相互作用（MRCI）计算检验是可靠的。我们的研究结果表明，在所有三电子键中，没有一个价键结构本身具有任何键合作用。事实上，在每个价键结构中，三个电子保持泡利排斥，而整个键合能来自于两个或多个组成价键结构之间电荷转移引起的共振。因此，三电子键是电荷转移键（CSB）。通过计算三电子键的拉普拉斯算子（L）来探究CSB的特征。因此，与电子对键中的CSB非常相似，例如F或[1.1.1]丙烷中的中心键，这里L也是正的，从而表明由于三电子价键结构中的泡利排斥，共享密度具有过剩的动能。三电子键的RE值总是大于相应的键能。对于双π-(3e)键合物种，RE非常大，超过100 kcal/mol。因此，可以得出结论，σ-和π-三电子键与双电子CSB一起在CSB家族中找到了它们的自然位置，它们具有相同的能量和拓扑特征。本文还提出了三电子CSB的实验表现/测试方法。
