Excited-State Aromaticity Reversals in Möbius Annulenes

Area of Science:

• Theoretical Chemistry
• Quantum Chemistry
• Organic Chemistry

Background:

• Möbius annulenes are cyclic conjugated systems with a twist, exhibiting unique aromaticity properties.
• Baird's rule predicts antiaromaticity for 4n systems and aromaticity for 4n+2 systems in triplet excited states.
• Understanding electronic states is crucial for predicting molecular stability and reactivity.

Purpose of the Study:

• To investigate the aromaticity of Möbius annulenes in different electronic states (ground, triplet, singlet excited).
• To test the hypothesis that Baird's rule criteria are reversed for Möbius annulenes in excited states.
• To examine the electronic properties of the cyclononatetraenyl cation (C9H9+) across its S0, T1, and S1 states.

Main Methods:

• Utilized state-optimized complete-active-space self-consistent field (CASSCF) wave functions.
• Employed gauge-including atomic orbitals (GIAO) for accurate calculations.
• Analyzed isotropic magnetic shielding isosurfaces to determine aromaticity.

Main Results:

• The ground state (S0) of the cyclononatetraenyl cation (C9H9+) was found to be aromatic.
• The lowest triplet (T1) and first singlet excited (S1) states of C9H9+ were determined to be antiaromatic.
• These findings support a reversal of Baird's rule for Möbius annulenes in excited electronic states.

Conclusions:

• Möbius annulenes demonstrate reversed aromaticity behavior in excited states compared to their ground states.
• The cyclononatetraenyl cation serves as a model system illustrating this phenomenon.
• This study advances the understanding of aromaticity in electronically excited organic molecules.