Cyclo[
n
]carbons (
n
= 5, 7, 9,
..., 29) composed from an odd number of carbon atoms are studied computationally
at density functional theory (DFT) and
ab initio
complete
active space self-consistent field (CASSCF) levels of theory to get
insight into their electronic structure and aromaticity. DFT calculations
predict a strongly delocalized carbene structure of the cyclo[
n
]carbons and an aromatic character for all of them. In
contrast, calculations at the CASSCF level yield geometrically bent
and electronically localized carbene structures leading to an alternating
double aromaticity of the odd-number cyclo[
n
]carbons.
CASSCF calculations yield a singlet electronic ground state for the
studied cyclo[
n
]carbons except for C
25
, whereas at the DFT level the energy difference between the lowest
singlet and triplet states depends on the employed functional. The
BHandHLYP functional predicts a triplet ground state of the larger
odd-number cyclo[
n
]carbons starting from
n
= 13. Current-density calculations at the BHandHLYP level
using the CASSCF-optimized molecular structures show that there is
a through-space delocalization in the cyclo[
n
]carbons.
The current density avoids the carbene carbon atom, leading to an
alternating double aromaticity of the odd-number cyclo[
n
]carbons satisfying the antiaromatic [4k+1] and aromatic [4k+3] rules.
C
11
, C
15
, and C
19
are aromatic and
can be prioritized in future synthesis. We predict a bond-shift phenomenon
for the triplet state of the cyclo[
n
]carbons leading
to resonance structures that have different reactivity toward dimerization.