Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
Department for Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
Nat Chem. 2017 Apr;9(4):369-373. doi: 10.1038/nchem.2675. Epub 2016 Nov 28.
How molecules in solution form crystal nuclei, which then grow into large crystals, is a poorly understood phenomenon. The classical mechanism of homogeneous crystal nucleation proceeds via the spontaneous random aggregation of species from liquid or solution. However, a non-classical mechanism suggests the formation of an amorphous dense phase that reorders to form stable crystal nuclei. So far it has remained an experimental challenge to observe the formation of crystal nuclei from five to thirty molecules. Here, using polyoxometallates, we show that the formation of small crystal nuclei is observable by cryogenic transmission electron microscopy. We observe both classical and non-classical nucleation processes, depending on the identity of the cation present. The experiments verify theoretical studies that suggest non-classical nucleation is the lower of the two energy pathways. The arrangement in just a seven-molecule proto-crystal matches the order found by X-ray diffraction of a single bulk crystal, which demonstrates that the same structure was formed in each case.
溶液中的分子如何形成晶核，然后晶核生长成大晶体，这是一个理解得很差的现象。均相晶核形成的经典机制是通过液体或溶液中物种的自发随机聚集进行的。然而，一种非经典机制表明形成无定形致密相，然后重新有序形成稳定的晶核。到目前为止，观察从五到三十个分子形成晶核仍然是一个实验挑战。在这里，我们使用多金属氧酸盐表明，小晶核的形成可以通过低温透射电子显微镜观察到。我们观察到了经典和非经典的成核过程，这取决于存在的阳离子的性质。实验验证了理论研究，表明非经典成核是两种能量途径中较低的一种。只有七个分子的原晶的排列与单个大块晶体的 X 射线衍射所发现的排列相匹配，这表明在每种情况下都形成了相同的结构。
