双碳分子？！

    2020年发表在Nature上的文章Room-temperature chemical synthesis of C2 报道了在室温下首次合成双碳分子——之前的工作主要是在电弧中捕捉这些分子的存在。

    通过分子轨道理论，我们计算出这种分子的键级为二，这是由于2s的同相与反相重叠产生的成键轨道和反键轨道相互抵消。当然，由于sp混杂的存在，键级并不是精准的2，但是大致上不会偏差太大。

    关于双碳分子最早的化学捕集是利用丙酮、乙醛等化合物，对电弧中的碳分子进行原位低温反应，同位素标记显示这种分子具有一定的自由基性质——但是在分子轨道理论中却并未体现出双自由基的任何迹象。另一种利用环辛四烯的捕集方法是通过产生的芳构化产物——蒽来进行机理确证。

    四重键模型是在价键理论的基础上建立起来的，包括一个乙炔骨架和一对呈现弱相互作用的v键：

   事实上，之前已经实现了非游离的双碳分子配合物的制备，主要是通过CAAC（环烷基氨基卡宾）或卡宾、叶立德（ylide）等，但是这样产生的双碳分子已经失去了原有的化学性质。这种配合物较为稳定，能够制备并且还能进行两步的氧化还原反应。 

    利用氟离子切掉TMS保护基，接着四氟硼酸根离去，TMS产生的负电荷消除碘苯，从而形成游离的双碳分子。双碳分子能够自发的进行自下而上的聚合反应，产生石墨烯、碳纳米管和碳球等结构。

    事实上，通过量子化学计算，可以推断在双碳分子中存在四重键，但是并不是在乙炔分子中衍生的，而是具有两个Π键，两个σ键，但是两个σ键并不是简并的，一个较强，另一个较弱，从而使它呈现双自由基的性质。双碳分子的键常数介于双键和叁键之间，这是由于Pauli排斥削弱了键强度。

    由于篇幅所限（其实是最近太忙啦），更多详细过程请参考：

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