Shaik Sason, Danovich David, Braida Benoit, Hiberty Philippe C
Institute of Chemistry and, The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
UPMC Université Paris 06, CNRS UMR 7616, Laboratoire de Chimie Théorique, C. 137, 4 Place Jussieu, 75252, Paris Cedex 05, France.
Chemistry. 2016 Mar 14;22(12):4116-28. doi: 10.1002/chem.201600011. Epub 2016 Feb 16.
Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C2 is bonded by a quadruple bond that can be distinctly characterized by valence-bond (VB) calculations. We demonstrate that the quadruply-bonded structure determines the key observables of the molecule, and accounts by itself for about 90% of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply-bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ-type bond, the bond strength of which is estimated as 17-21 kcal mol(-1). Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol(-1), respectively, above the quadruply-bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply-bonded model is underscored by "predicting" the properties of the (3)Σ+u state. C2's very high reactivity is rooted in its fourth weak bond. Thus, carbon and first-row main elements are open to quadruple bonding!
自从刘易斯描绘出乙炔的三键以来，三键一直被视为主族元素多重键的最高限度。在此我们表明，C₂ 通过四重键相连，这可以通过价键（VB）计算清晰地表征出来。我们证明，四重键结构决定了该分子的关键可观测性质，其自身就占了该分子键解离能的约90%，还决定了其键长和力常数。四重键结构由两个强π键、一个强σ键和一个较弱的第四σ型键组成，其键强度估计为17 - 21千卡/摩尔⁻¹。具有双键的替代VB结构；即两个π键或一个π键和一个σ键，分别比四重键结构高出129.5和106.1千卡/摩尔⁻¹，并且如果有通过配位σ键改善其双键的自由度，它们会坍缩为后者结构。通过“预测”³Σ⁺u态的性质，四重键模型的实用性得到了强调。C₂ 极高的反应活性源于其第四个弱键。因此，碳和第一周期主族元素可以形成四重键！
