The arithmetic of aromaticity

Abstract

In this article, the authors explore an aspect of conjugated systems that have received little attention, namely polycyclic hydrocarbons.

The crystal structures of two chalcones: (2E)-1-(5-chlorothiophen-2-yl)-3-(2-methylphenyl)prop-2-en-1-one and (2E)-1-(anthracen-9-yl)-3-[4-(propan-2-yl)phenyl]prop-2-en-1-one

“…Within each of the anthracene units, the distances Cn11-Cn12, Cn13-Cn14, Cn15-Cn16 and Cn17-Cn18 are very much shorter than the other C-C bonds in these units, while the C-C distances in the central rings show rather little variation. These observations are consistent with an electronic structure for the anthracene units where a central ring displaying aromatic delocalization is flanked by two isolated diene units (Glidewell & Lloyd, 1984,1986).…”

A chain of π-stacked molecules in 4-(2-chlorophenyl)pyrrolo[1,2-a]quinoxaline and a hydrogen-bonded sheet in (4RS)-4-(1,3-1,3-benzodioxol-6-yl)-4,5-dihydropyrrolo[1,2-a]quinoxaline

“…At the same time, the bond lengths for the fused carbocyclic ring of (I), but not for that of (II), indicate some slight bond fixation in the C6-C7 and C8-C9 bonds, while in the central fused ring of (I) there is a clearly localized double bond at C4-N5. The overall pattern of the distances in (I) indicates the presence of 6 circuits in the two outer rings of the fused system and a localized double bond in the central ring, analogous to the electronic structure of phenanthrene (Glidewell & Lloyd, 1984, 1986. This pattern of distances in (I) is fairly similar to that found for (III) (Castillo et al, 2010).…”

Pseudomerohedral twinning, pseudosymmetry and complex hydrogen-bonded sheets in 2-methoxy-4-(pyrrolo[1,2-a]quinoxalin-4-yl)phenol

“…In the carbocyclic rings, the bond distances support aromatic delocalization, but with some slight bond fixation in the Cx6-Cx7 and Cx8-Cx9 bonds, while in the central rings there are clearly localized double bonds at the Cx4-Nx5 bonds. Overall, therefore, the pattern of two 6 circuits in the two outer rings of the fused system, with a localized double bond in the central ring, is reminiscent of the electronic structure of phenanthrene and related molecules (Glidewell & Lloyd, 1984, 1986.…”