posted on 2024-02-08, 06:05authored byYang Wang, Yi Zhou, Ke Du
With recent breakthroughs and advances
in synthetic chemistry,
carbon nanobelts (CNBs) have become an emerging hot topic 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,
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) involve 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 the cylindrical carbon framework, and the
former effect prevails over the latter. For nonstandard CNBs, a third
stability factor, the H···H repulsion in the benzo[c]phenanthrene-like motifs, is also shown to be 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 most stable CNB isomers determined can be considered 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 analogues.