Enumeration, Nomenclature and Stability Rules of Carbon Nanobelts

With the recent breakthroughs and advances in synthetic chemistry, carbon nanobelts (CNBs) have become an emerging hot spot in chemistry and materials science. Owing to their unique molecular structures, CNBs have intriguing properties with applications in synthetic materials, host–guest chemistry, optoelectronics, and so on. Although a considerable number of CNBs with diverse forms have been synthesized to date, no systematic nomenclature is available yet for this important family of macrocycles. Moreover, little is known about the detailed isomerism of CNBs, which, in fact, exhibits greater complexity than that of carbon nanotubes. The copious variety of CNB isomers, along with the underlying structure–property relationships, bears fundamental relevance to the ongoing design and synthesis of novel nanobelts. In this paper, we propose an elegant approach to systematically enumerate, classify, and name all possible isomers of CNBs. Besides the simplest, standard CNBs defined by chiral indices (n, m), the nonstandard CNBs (n, m, l) involves an additional winding index l. Based on extensive quantum chemical calculations, we present a comprehensive study of the relative isomer stability of CNBs containing up to 30 rings. A simple Hückel-based model with a high predictive power reveals that the relative stability of standard CNBs is governed by the π stabilization and the strain destabilization induced by cylindrical carbon framework, and the former effect prevails the latter. For nonstandard CNBs, a third stability factor, the H...H repulsion in the benzo[c]phenanthrene-like motifs, is shown to be also important and can be incorporated into the simple quantitative model. In general, lower-energy CNB isomers have a larger HOMO–LUMO gap, suggesting that their thermodynamic stability coincides with kinetic stability. The determined most stable CNB isomers can be considered as the optimal targets for future synthesis. These results lay an initial foundation and provide a useful theoretical tool for further research on CNBs and related analogs.