Aromaticity Reversal Induced by Vibrations in Cyclo[16]carbon

The odd-number cyclo[13]carbon and its dimer, cyclo[26]carbon

“…However, inducing stress owing to a finite ring size lifts the energy degeneracy of in-plane and out-of-plane systems. The out-of-plane orbital energies become slightly lower than their in-plane counterparts ( 9 , 14 ). Therefore, in the absence of both BLA and BAA ( D 13h geometry), the in-plane orbitals A ’ and B ’ would be singly occupied, whereas their out-of-plane counterparts A " and B " would be doubly occupied, resulting in a triplet ground state 3 |11 22> (Fig.…”

“…According to a double particle-on-a-ring model, C 13 has four π orbitals near the Fermi energy, two of the in-plane π system and two of the out-of-plane π system, as shown in Fig. 3A ( 9 , 14 ). Neutral C 13 has six electrons in these four frontier orbitals, and different occupation of these orbitals results in several possible electronic states.…”

“…S21) indicated that C 13 in its 3 |21 21> ground state sustains an antiaromatic ring current of about –18 nA/T corresponding to a positive zz component of the nucleus-independent chemical shift calculated 2 Å above the ring centre [NICS(2) zz = +22.5 ppm], which is slightly weaker than the –25 nA/T current reported in the doubly (Hückel) antiaromatic C 16 ( 12 , 14 ) in which both π systems produce an antiaromatic current. The 3 |21 21> ground state of C 13 , in contrast to the strongly antiaromatic 1 |22 20> excited state ( 13 , 14 ) (fig. S21), cannot be discussed in terms of Hückel or Baird aromaticity rules because of an odd number of electrons in both π systems.…”

“…Cyclocarbons are relatively small, and their electronic structure resembles that of a particle on a ring and can be rationalized with textbook physics ( 5 – 7 ). However, the many energy-degenerate orbitals and the possibility of multiple Jahn-Teller distortions render calculations extremely demanding ( 7 – 14 ), making cyclocarbons excellent model systems for benchmarking theoretical approaches ( 15 ). Moreover, their monocyclic structures make them interesting systems in the context of aromaticity ( 1 – 4 , 11 – 14 ).…”

“…However, the many energy-degenerate orbitals and the possibility of multiple Jahn-Teller distortions render calculations extremely demanding ( 7 – 14 ), making cyclocarbons excellent model systems for benchmarking theoretical approaches ( 15 ). Moreover, their monocyclic structures make them interesting systems in the context of aromaticity ( 1 – 4 , 11 – 14 ). For many years, experimental investigations of cyclocarbons were limited to studies in the gas phase ( 16 , 17 ) or in a solid matrix ( 18 ).…”