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Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles
- 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.
- Subjects :
- Isodesmic reaction
010405 organic chemistry
Chemistry
Aromaticity
General Chemistry
Annulene
010402 general chemistry
Ring (chemistry)
01 natural sciences
Biochemistry
Porphyrin
Catalysis
0104 chemical sciences
chemistry.chemical_compound
Colloid and Surface Chemistry
Computational chemistry
Molecule
Density functional theory
Antiaromaticity
Subjects
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