Aromaticity and Antiaromaticity Reversals between the Electronic Ground State and the Two Lowest Triplet States of Thiophene.

ABSTRACT: It is shown, by examining the variations in off-nucleus isotropic magnetic shielding around a molecule, that thiophene which is aromatic in its electronic ground state (S₀) becomes antiaromatic in its lowest triplet state (T₁) and then reverts to being aromatic in T₂. Geometry relaxation has an opposite effect on the aromaticities of the ππ* vertical T₁ and T₂: The antiaromaticity of T₁ is reduced whereas the aromaticity of T₂ is enhanced. The shielding picture around T₂ is found to closely resemble those around certain second singlet ππ* excited states (S₂), for example, those of benzene and cyclooctatetraene, thought to be "strongly aromatic" because of their very negative nucleus-independe

SUBMITTER: Cummings E 

PROVIDER: S-EPMC11747585 | biostudies-literature | 2025 Jan

REPOSITORIES: biostudies-literature