Grupo de Modelización y Diseño Molecular, Dpto. de Química Orgánica, c/ Severo Ochoa s/n, Facultad de Ciencias, Universidad de Granada, 18071-Granada, Spain.
Phys Chem Chem Phys. 2011 Jul 28;13(28):12844-57. doi: 10.1039/c1cp20672a. Epub 2011 Jun 20.
Previous investigations have revealed that even long carbon nanotubes (CNTs) retain bond patterns that are characterized by the localization of Clar rings. Even for CNTs with 10 nm length, an alternated, oscillating structure of Clar and Kekulé patterning was also found, indicating that these arrangements may possibly persist for even longer nanotubes, given that they are finite. In the present work, we perform a detailed and comprehensive theoretical study of this phenomenon, in order to find the causes that give rise to these patterns. A complete set of CNTs with different chiralities, diameters (up to 2 nm), lengths (up to 10 nm) and endings (capped, uncapped, and tailored endings) was considered for such purposes. The results indicate that the Clar patterning appears not only on armchair CNTs, but also on those with chiral angle values close to 30°, and this results in a stabilization of the structure, when compared with the uniform, zigzag CNTs. This stabilizing effect points to the causes that underlie the three Nakamura CNT types, resulting as the superposition of structures with a maximal number of Clar rings. Although there is a strict dependence on the border shape, the main cause of the bond patterning in long tubes is to be found in the intrinsic wrapping of each CNT, because the type and number of oscillations present in the longest structures do not depend on the particular length. Nevertheless, the three Nakamura types of armchair tubes appear to subsist beyond the appearance of oscillations, because each of these sets evolves in a different manner, and energy properties that link them together. Apart from the geometry, Clar patterning was investigated through NICS (Nucleus Independent Chemical Shifts) measures, which reveal a connection between the Clar rings and a local concentration of aromaticity.
先前的研究表明，即使是长的碳纳米管（CNTs）也保留了以 Clar 环局域化为特征的键图案。即使对于长度为 10nm 的 CNTs，也发现了 Clar 和 Kekulé 图案交替、振荡的结构，这表明这些排列可能在更长的纳米管中持续存在，因为它们是有限的。在目前的工作中，我们对这种现象进行了详细和全面的理论研究，以找出导致这些图案的原因。为了达到这个目的，我们考虑了一系列不同手性、直径（高达 2nm）、长度（高达 10nm）和末端（帽状、无帽状和裁剪末端）的 CNTs。结果表明，Clar 图案不仅出现在扶手椅 CNTs 上，也出现在手性角接近 30°的 CNTs 上，这导致了结构的稳定化，与均匀的锯齿状 CNTs相比。这种稳定化效应指向了三种 Nakamura CNT 类型的基础原因，是由具有最大数量 Clar 环的结构的叠加所导致的。尽管存在严格的边界形状依赖性，但长管中键图案的主要原因在于每个 CNT 的内在包裹，因为最长结构中存在的振动类型和数量不依赖于特定的长度。然而，三种 Nakamura 类型的扶手椅管似乎在出现振动之后仍然存在，因为这些集合中的每一个都以不同的方式演变，并且能量特性将它们联系在一起。除了几何形状，Clar 图案还通过核独立化学位移（NICS）测量进行了研究，这揭示了 Clar 环与局部芳香性之间的联系。
