Cummings Edward, Karadakov Peter B
Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
Chemphyschem. 2025 Jan 2;26(1):e202400758. doi: 10.1002/cphc.202400758. Epub 2024 Nov 5.
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.
通过研究分子周围离核各向同性磁屏蔽的变化表明，噻吩在其电子基态（S）下是芳香性的，在其最低三重态（T）下变为反芳香性，然后在T态又恢复为芳香性。几何弛豫对ππ垂直T态和T态的芳香性有相反的影响：T态的反芳香性降低，而T态的芳香性增强。发现T态周围的屏蔽情况与某些第二单重态ππ激发态（S）周围的屏蔽情况非常相似，例如苯和环辛四烯的激发态，由于它们具有非常负的与核无关化学位移（NICS）值，被认为是“强芳香性”的。有人认为，虽然NICS值能正确地反映单一电子态势能面上芳香性的变化，但从理论上讲，用NICS值来定量比较不同电子态的芳香性是不合理的，应该避免。“强芳香性”的S态和T态应简单地称为“芳香性”，因为对这些态与相应S态性质的详细比较并未表明其具有更高程度的芳香性。
