Department of Chemistry , Brandeis University , Waltham , Massachusetts 02453 , United States.
Department of Chemistry and Chemical Biology, Baker Laboratory , Cornell University , Ithaca , New York 14853-1301 , United States.
J Am Chem Soc. 2018 Jun 20;140(24):7658-7666. doi: 10.1021/jacs.8b03733. Epub 2018 Jun 8.
Carbon nanothreads are a new type of one-dimensional sp-carbon nanomaterial formed by slow compression and decompression of benzene. We report characterization of the chemical structure of C-enriched nanothreads by advanced quantitative, selective, and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy. The width of the NMR spectral peaks suggests that the nanothread reaction products are much more organized than amorphous carbon. In addition, there is no evidence from NMR of a second phase such as amorphous mixed sp/sp-carbon. Spectral editing reveals that almost all carbon atoms are bonded to one hydrogen atom, unlike in amorphous carbon but as is expected for enumerated nanothread structures. Characterization of the local bonding structure confirms the presence of pure fully saturated "degree-6" carbon nanothreads previously deduced on the basis of crystal packing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise between 20% and 45% of the sample. Furthermore, C-C spin exchange experiments indicate that the length of the fully saturated regions of the threads exceeds 2.5 nm. Two-dimensional C-C NMR spectra showing bonding between chemically nonequivalent sites rule out enumerated single-site thread structures such as polytwistane or tube (3,0) but are consistent with multisite degree-6 nanothreads. Approximately a third of the carbon is in "degree-4" nanothreads with isolated double bonds. The presence of doubly unsaturated degree-2 benzene polymers can be ruled out on the basis of C-C NMR with spin exchange rate constants tuned by rotational resonance and H decoupling. A small fraction of the sample consists of aromatic rings within the threads that link sections with mostly saturated bonding. NMR provides the detailed bonding information necessary to refine solid-state organic synthesis techniques to produce pure degree-6 or degree-4 carbon nanothreads.
碳纳米线是一种新型的一维 sp 碳纳米材料，由苯的缓慢压缩和解压形成。我们通过先进的定量、选择性和二维固态核磁共振 (NMR) 实验以及红外 (IR) 光谱对富碳纳米线的化学结构进行了表征。NMR 光谱峰的宽度表明，纳米线反应产物比无定形碳更加有序。此外，NMR 没有无定形混合 sp/sp 碳等第二相的证据。光谱编辑表明，几乎所有的碳原子都与一个氢原子键合，这与无定形碳不同，但与枚举的纳米线结构的预期情况一致。局部键合结构的表征证实了纯完全饱和的“度-6”碳纳米线的存在，这是以前基于晶体堆积考虑从衍射和透射电子显微镜推断出来的。这些完全饱和的线占样品的 20%到 45%之间。此外，C-C 自旋交换实验表明，完全饱和区的纳米线长度超过 2.5nm。二维 C-C NMR 谱显示化学上不等价的键合位点之间的键合排除了枚举的单站点线程结构，如多扭曲烷或管 (3,0)，但与多站点度-6 纳米线一致。大约三分之一的碳存在于具有孤立双键的“度-4”纳米线中。根据 C-C NMR 并通过旋转共振和 H 去耦来调节自旋交换速率常数，可以排除具有双键的度-2 苯聚合物的存在。样品的一小部分由连接具有主要饱和键合的部分的线程中的芳环组成。NMR 提供了必要的详细键合信息，以改进固态有机合成技术，以生产纯度-6 或度-4 碳纳米线。
