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

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 (S) becomes antiaromatic in its lowest triplet state (T) and then reverts to being aromatic in T. Geometry relaxation has an opposite effect on the aromaticities of the ππ* vertical T and T: The antiaromaticity of T is reduced whereas the aromaticity of T is enhanced. The shielding picture around T is found to closely resemble those around certain second singlet ππ* excited states (S), for example, those of benzene and cyclooctatetraene, thought to be "strongly aromatic" because of their very negative nucleus-independent chemical shift (NICS) values. It is argued that while NICS values correctly follow the changes in aromaticity along the potential energy surface of a single electronic state, the use of NICS values for the purpose of quantitative comparisons between the aromaticities of different electronic states cannot be justified theoretically and should be avoided. "Strongly aromatic" S and T states should be referred to simply as "aromatic" because detailed comparisons between the properties of these states and those of the corresponding S states do not suggest higher levels of aromaticity.