Aromaticity rules

Abstract

Standfirst: In 1931, Erich Hückel published a landmark paper and the seed of the nowfamous 4n+2 rule for aromaticity in annulenes that bears his name. Electron counting has since been extended to other classes of compounds, resulting in a multitude of rules aiming to capture the concept of aromaticityand its impact in chemistry.

Planar inorganic five-membered heterocycles with σ + π dual aromaticity in both S0and T1states

“…Aromaticity can be qualitatively understood/predicted by the rule of thumb. 6,7 For a cyclic molecule, its aromaticities in the lowest singlet state and the lowest triplet state are governed by the classical Hückel's 4 n + 2 π-electron rule 8 ( n is an integer) and Baird's 4 n π-electron rule, 9 respectively. The two rules imply that a ring system cannot exhibit singlet- and triplet-state aromaticities simultaneously.…”

The Curious Case of Low-lying States in Non-linear Polyaromatic Hydrocarbons

“…3). 33,34 There are two Clar structures that can be formed for phenanthrene -one with two Clar sextets and the other with a single Clar sextet. On the other hand, anthracene can have structures with a single Clar sextet only.…”

Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes

“…[6][7][8][9][10] However, utilizing these properties of is hampered by the inherent instability of such systems. Annellation of aromatic rings to (polycyclic) antiaromatic molecules [11][12][13][14] can drastically stabilize the reactive cores thermodynamically by adding local aromaticity to the system while retaining the peculiar properties ascribed to antiaromaticity. 9,[15][16][17][18][19] For example, benzannellation to the highly antiaromatic unstable pentalene 20 leads to the benchtop-stable dibenzo[a,e]pentalene (DBP), [21][22][23][24][25][26] despite its 4n -electron count (Figure 1a).…”