Excited-State Aromaticity Reversals in Möbius Annulenes

It
is suggested that Möbius annulenes follow a rule similar
to Baird’s rule such that the 4n and 4n + 2 criteria for Möbius electronic ground-state
aromaticity and antiaromaticity are reversed in the lowest triplet
and first singlet excited electronic states. Support comes from an
investigation of aromaticity in the ground (S0), lowest
triplet (T1), and first singlet excited (S1)
electronic states of the Möbius-aromatic cyclononatetraenyl
cation, C9H9+, using isotropic magnetic shielding isosurfaces calculated
with state-optimized complete-active-space self-consistent field wave
functions constructed from gauge-including atomic orbitals. Examination
of these isosurfaces demonstrates that while the S0 state
of C9H9+ is aromatic, the T1 and S1 states are
antiaromatic.