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Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

Authors :
Lara Tejerina
Michael Jirásek
Michel Rickhaus
Harry L. Anderson
Source :
Journal of the American Chemical Society. 143:2403-2412
Publication Year :
2021
Publisher :
American Chemical Society (ACS), 2021.

Abstract

Enhanced thermodynamic stability is a fundamental characteristic of aromatic molecules, yet most previous studies of aromatic stabilization energy (ASE) have been limited to small rings with up to 18 π-electrons. Here we demonstrate that ASE can be detected experimentally in π-conjugated porphyrin nanorings with Hückel circuits of 76-108 π-electrons. This conclusion is supported by analyzing redox potentials to calculate the energy change for isodesmic reactions that convert an aromatic ring to an antiaromatic ring or vice versa. It is also supported by analyzing the energy barriers to conformational equilibria that disrupt aromaticity in the transition state. Both types of experiment indicate that cationic porphyrin nanorings display ASEs of 1-5 kJ mol-1. Density functional theory calculations reproduce the results for both types of experiment and predict ASEs in the range of 1-16 kJ mol-1. The experimental ASEs in porphyrin nanorings are compared with an experimental ASE of [18]annulene of ∼11 kJ mol-1, deduced from analysis of the energy barriers to conformational equilibria in [16], [18], and [20]annulene. Calculated energies of isodesmic reactions give an ASE of ∼37 kJ mol-1 in [18]annulene. This work contributes to a fundamental understanding of aromaticity in large macrocycles.

Details

ISSN :
15205126 and 00027863
Volume :
143
Database :
OpenAIRE
Journal :
Journal of the American Chemical Society
Accession number :
edsair.doi.dedup.....8b122d5908c1dc65afa58bf761bccee8
Full Text :
https://doi.org/10.1021/jacs.0c12845