Connecting and combining rules of aromaticity. Towards a unified theory of aromaticity

Abstract

Most of the archetypal aromatic compounds present high symmetry and have degenerate highest‐occupied molecular orbitals. These orbitals can be fully occupied resulting in a closed‐shell structure or can be same‐spin half‐filled. This closed‐shell or same‐spin half‐filled electronic structure provides an extra stabilization and it is the origin of several rules of aromaticity such as the Hückel 4N + 2 rule, the lowest‐lying triplet excited state 4N Baird rule, the 4N rule in Möbius‐type conformation, the 4N + 2 Wade–Mingos rule in closo boranes or the 2(N + 1)2 Hirsch rule in spherical aromatic species. Combinations of some of these rules of aromaticity can be found in some particular species. Examples of these combinations will be discussed and the validity of some of these rules will be assessed. Moreover, it is possible to establish connections with some of these rules of (anti)aromaticity and, therefore, they can be partially generalized, which represents a step forward to a future unified theory of aromaticity. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Density Functional Theory Software > Quantum Chemistry

The symmetry principle of antiaromaticity

“…. , x n ) (1) with the n the number of electrons, the electronic charge −e, the electron mass m e , and the ground state wave function Ψ 0 that depends on spatial coordinates of the i-th electron r i and its spin coordinate s i which are combined to the space-spin coordinate x i and the (purely imaginary) first order magnetic response contribution to the wave function…”

“…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 also experimental evidence suggests a specific type of electronic structure. One often encounters this in the form of electron count rules, orbital occupation patterns or electronic resonance 1…”

Magnetically induced currents and aromaticity in ligand-stabilized Au and AuPt superatoms

“…This classical ring current is called a diatropic current, increasing the shielding of the part of the object inside the current ring against the effect of the external field. Systems with strong diatropic currents are called aromatic 2 , 3 . Antiaromatic systems have a strong non-classical current component (paratropic current) that tends to create an internal magnetic field parallel to the external one.…”

Estudo teórico da aromaticidade dos compostos heterocíclicos pirrol, furano e tiofeno no Ensino Médio

“…A aromaticidade é um conceito fundamental na Química Orgânica, que descreve a estabilidade especial de moléculas ciclicamente conjugadas com um certo número de elétrons (SOLÀ, 2018). Por outro lado, a antiaromaticidade descreve uma desestabilização.…”