Electronic Structures of [*n*]-Cyclacenes (*n* = 6-12) and Short,
Hydrogen-Capped, Carbon Nanotubes

Sadowsky, D.; McNeill, K.; Cramer, C. J.

*Faraday Discuss.*
**2010**, *145*, 507
(doi:10.1039/b906882a).

Density functional (M06-L and B3LYP) and multiconfigurational second-order
perturbation (CASPT2) theories are applied to [*n*]-cyclacenes having
*n* =
6-12 in order to assess their strain energies and the degree of di- or
polyradical character inherent in their electronic structures. In the case
of density functional theory, a broken symmetry approach must be employed
and the sensitivity of the results to choice of functional and
broken-symmetry protocol is explored. Viewing the [*n*]-cyclacenes as
monomeric building blocks which may be joined to grow finite length
(*n*,0)
single-walled carbon nanotubes, density functional calculations are further
employed to explore changes in electronic structure associated with
increasing nanotube length. Spin-state energy gaps are predicted to
decrease both with increasing cyclacene size and with increasing nanotube
length; [*n*]-cyclacenes with odd values of *n* are predicted to develop
polyradical character for smaller *n* than is the case for even values
*n*.