Abstract
In this paper, topological resonance energy (TRE) methods were used to describe the global aromaticity of nitrogen confused porphyrin (NCP) isomers. The TRE results show that all NCP isomers exhibit lower aromaticity than the normal porphyrins, and their aromaticity decreases as the number of confused pyrrole rings in the molecule increases. In the NCPs, global aromaticity decreases as the distance between the nitrogen atoms increases. The bond resonance energy (BRE) and circuit resonance energy (CRE) indices were applied to study local aromaticity and conjugated pathways. Both the BRE and CRE indices revealed that individual pyrrolic subunits maintain their strong aromatic character and are the main source of global aromaticity. Ring currents (RC) were analysed using the Hückel–London model. RC results revealed that the macrocyclic electron conjugation pathway is the main source of diatropicity. As the number of confused pyrrole rings in the molecule increases, its diatropicity gradually decreases. In the confused pyrrole rings of the NCP isomers, the diatropic RC passing through the β-positions is always weaker than that passing through the inner sections. This is unrelated to the location of the protonated or non-protonated nitrogen atom at the periphery of the molecule and must be ascribed to the unique properties of the confused pyrrole rings.
Highlights
The aromaticity and main conjugation pathways of porphyrins and the related macrocycle have been subjects of considerable interest for many years [1,2,3,4,5]
We report the global aromaticity of nitrogen confused porphyrin (NCP) isomers and compare these with the normal porphyrins by the use of the topological resonance energy (TRE) method
In order to further understand the effects of nitrogen atoms on global aromaticity, we have studied the aromaticity of subunits of the NCP isomers
Summary
The aromaticity and main conjugation pathways of porphyrins and the related macrocycle have been subjects of considerable interest for many years [1,2,3,4,5]. N-confused porphyrins (NCPs) are a class of porphyrin isomers in which one or more of the core nitrogen atoms faces outward from the macrocycle and a CH unit faces inward toward the core [6,7]. One or more pyrrole rings are connected to the meso carbon atoms at the α and β positions. Many reports have demonstrated that these compounds possess properties quite different from normal porphyrins in terms of their aromatic character and in their royalsocietypublishing.org/journal/rsos R.
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