Aromaticity of Even-Number Cyclo[n]carbons (n = 6–100)

Abstract

The recently synthesized cyclo[18]carbon molecule has been characterized in a number of studies by calculating electronic, spectroscopic, and mechanical properties. However, cyclo[18]carbon is only one member of the class of cyclo[ n ]carbons—standalone carbon allotrope representatives. Many of the larger members of this class of molecules have not been thoroughly investigated. In this work, we calculate the magnetically induced current density of cyclo[ n ]carbons in order to elucidate how electron delocalization and aromatic properties change with the size of the molecular ring ( n ), where n is an even number between 6 and 100. We find that the Hückel rules for aromaticity (4 k + 2) and antiaromaticity (4 k ) become degenerate for large C n rings ( n > 50), which can be understood as a transition from a delocalized electronic structure to a nonaromatic structure with localized current density fluxes in the triple bonds. Actually, the calculations suggest that cyclo[ n ]carbons with n > 50 are nonaromatic cyclic polyalkynes. The influence of the amount of nonlocal exchange and the asymptotic behavior of the exchange–correlation potential of the employed density functionals on the strength of the magnetically induced ring current and the aromatic character of the large cyclo[ n ]carbons is also discussed.

Remarkable Size Effect on Photophysical and Nonlinear Optical Properties of All‐Carboatomic Rings, Cyclo[18]carbon and Its Analogues

“…For carbon rings with different sizes, electronic properties including the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), singlet‐triplet splitting, ionization potential, and electron affinity for cyclo[ n ]carbons ( n =10–100) were previously studied using thermally‐assisted‐occupation density functional theory (TAO‐DFT) [25] . Aromatic behavior and molecular magnetizability of some cyclo[ n ]carbons ( n up to 100) were also reported by Baryshnikov, Valiev, and their collaborators [26,27] . Optical properties, such as electronic absorption spectrum and (hyper)polarizability, are important molecular characteristics because they determine the possibility that molecules will eventually be used as practical optical materials.…”

Electronic spectra of positively charged carbon clusters—C2n+ (n = 6–14)

“…42 High resolution atomic force microscopy showed that C 18 formed in this way has a cyclic polyynic structure, as predicted by electronic structure calculations. 43 Ultimately, gas-phase electronic spectra of cyclocarbon cations reported in this paper should provide useful data for benchmarking theoretical approaches, which, in turn, should help guide synthetic condensed-phase studies.…”

Visualisation of Chemical Shielding Tensors (VIST) to Elucidate Aromaticity and Antiaromaticity**

“…[9][10][11][12] A new and exciting frontier is opened in terms of macrocycles and larger π-conjugated systems, in which local and global (anti)aromaticity can both play a role. Here, (anti) aromaticity is being studied in systems as diverse as nanographenes, [13,14] porphyrin nanorings, [15,16] carbon nanobelts, [17] cyclocarbon, [18] cycloparaphenylenes, [19][20][21] cycloparaphenylmethine, [22] paracyclophanetetraene, [23,24] norcorrole [25,26] and other porphyrinoids. [27,28] Several of these systems are interesting due to their remarkable capacity to stabilise multiply charged ions [20] making them promising candidates for organic battery electrodes.…”