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.