Abstract
Abstract Current definitions of aromaticity are purely phenomenological and relate symmetry, reactive stability and the occurrence of molecular diamagnetic response currents. The antithetical concept of antiaromaticity provides a connection between the contrary properties: structural instability or distortion out of higher symmetry, a small HOMO-LUMO gap, and paramagnetic response currents. We reveal the symmetry principle that is underlying antiaromaticity by showing an intimate and strict symmetry induced relation between these properties. The principle is mathematically rigorous and can be formulated like: First order (and related) Jahn-Teller distorted molecules out of non-cubic and non-icosahedral point groups are prone to induced paramagnetism in magnetic fields parallel to the main axis of symmetry. We show by the exemplary cases of cyclobutadiene, cyclcooctatetraene, pentalene and manganese trifluoride how this principle works and discuss this new perspective on antiaromaticity.
Highlights
IntroductionIt aims at an abstraction of different experimentally observed properties in a class of chemical compounds
Aromaticity is one of the most widely used chemical concepts
To show how these properties are related we use the following arguments –– Paramagnetic molecular response is determined by virtual electronic excitations of angular momentum symmetry, where the axis of rotation is parallel to the magnetic field. –– Ground state-excited state symmetries that give rise to virtual electronic excitations of rotational symmetry are connected with certain Jahn-Teller (JT) distortions out of specific molecular point group symmetries. –– These certain JT distortions are first order JT distortions and happen out of point group symmetries that are not cubic and not icoshedral
Summary
It aims at an abstraction of different experimentally observed properties in a class of chemical compounds These properties are typically a high structural or energetic stability, proneness to specific chemical reactions and susceptibility to induced diamagnetic currents when at the same time a theoretical description or experimental evidence suggests a specific type of electronic structure. A full quote of the definition is given in the Supplementary Information section To show how these properties are related we use the following arguments –– Paramagnetic molecular response is determined by virtual electronic excitations of angular momentum (or “rotational”) symmetry, where the axis of rotation is parallel to the magnetic field. First order and primoid second order JT distorted molecules out of non-isometric point groups are prone to induced paramagnetism in magnetic fields parallel to the main axis of symmetry. We start the results section with a derivation of a symmetry based “selection rule” for virtual electronic excitations determining paramagnetic response currents
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