Abstract
In this article, the topological resonance energy (TRE) method was used to investigate the global aromaticity of a set of [18]annulene-derived compounds which were obtained by replacing either two, four, or all six of the inner hydrogen atoms of [18]annulene with bridges (oxygen, imino-, sulfur, or combinations of the three). Our TRE results show that all the mono-bridged, di-bridged, and tri-bridged [18]annulenes are globally aromatic systems with positive TREs and show relatively larger aromaticity in comparison with the [18]annulene. The aromaticity of each compound was explained using the topological charge stabilization (TCS) rule. The bond resonance energy (BRE) and circuit resonance energy (CRE) methods were used to evaluate local aromaticity. Our BRE and CRE results show that incorporation of five-membered heterocyclic rings changes the main conjugation pathway of the bridged [18]-annulenes. The local aromaticities arising from the five-membered heterocyclic rings (6π) contribute strongly to global aromaticity. However, the ring current (RC), which arises from the 18π annulene-like pathway structures, is the primary determinant of the magnetic properties of the molecule. Our RC results are in good agreement with available 1H-NMR chemical shift data.
Highlights
Annulenes are monocyclic conjugated polyenes with alternating single and double bonds
Our bond resonance energy (BRE) and circuit resonance energy (CRE) results show that incorporation of fivemembered heterocyclic rings changes the main conjugation pathway of the bridged [18]-annulenes
BRE represents the contribution of a given p-bond to the topological resonance energy (TRE).23
Summary
Annulenes are monocyclic conjugated polyenes with alternating single and double bonds. Since Badger et al. rst reported the synthesis of [18]annulene trisulphide, bridged [18]annulenes have been the subject of much research in both theoretical and experimental organic chemistry These compounds belong to a family of polycyclic aromatic compounds. The determination of the main source of aromaticity is the most important task in the study of polycyclic compounds and has attracted much attention.. The determination of the main source of aromaticity is the most important task in the study of polycyclic compounds and has attracted much attention.19–22 In these compounds, every pathway contributes to a greater or lesser extent, to its global aromaticity.
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