The Stabilization Energy of [18] Annulene. A thermochemical determination

Oth, Jean F. M.; Bünzli, Jean‐Claude G.; Zélicourt, Yves de Julien de

doi:10.1002/hlca.19740570745

Cited by 22 publications

(23 citation statements)

References 13 publications

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“…This alternation in barrier height, from experimental NMR data, suggests that the ASE of [18]annulene is about 11 kJ mol -1 (compared with 36.9 kJ mol -1 from DFT calculations of hyperhomodesmotic reactions), which is significantly lower than previously reported estimates. 15,19,[20][21][22][23]25 To the best of our knowledge, this is the first study to provide experimental evidence for ASE in systems with Hückel electron counts greater than 18. Our results show that aromaticity has a small, but measurable, effect on the stability of large π-conjugated macrocycles.…”

Section: Discussionmentioning

confidence: 99%

Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

et al. 2021

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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 (DFT) calculations reproduce the results for both types of experiment, and predict ASEs in the range 1-16 kJ mol -1 . The experimental ASE in porphyrin nanorings are compared with an experimental ASE of [18]annulene of approximately 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 around 37 kJ mol -1 in [18]annulene. This work contributes towards fundamental understanding of aromaticity in large macrocycles.

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Section: Discussionmentioning

confidence: 99%

Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

et al. 2021

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“…The actual sample temperature may be substantially lower than 150". Since [18]annulene was known to thermally decompose in solution above 100" [5] many experiments were carried out to insure that the spectrum reported in Neglecting the out-of-plane deviations, a structure belonging to c*h symmetry (Fig. 2 in [ 11) can be obtained.…”

Section: Discussionmentioning

confidence: 99%

“…spectrum [3], of NMR. [4], and of thermochemical [5] measurements. On the other hand a structure of D j h symmetry was predicted by different authors on the basis of various calculations [6-121.…”

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The Photoelectron Spectrum of [18]Annulene

Baumann¹,

Bünzli²,

Oth³

1982

Helvetica Chimica Acta

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The PE. spectrum of [18]annulene has been measured and correlated with MO‐calculations. The experimental ionization energies can only be explained by computing the electronic states of the cation, that is by taking into account the electron correlation and reorganization in the ionic states. The results allow a discussion of the structure of the neutral molecule; they are consistent with a D6hpoint group of symmetry.

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“…This structure differs slightly from the D 3 structure (and still has alternating bond lengths), and not only has a lower energy, but it fi ts the NMR data better. The heat of formation of the compound is known to be 124.0 kcal/mol experimentally, 31 and the MM4 value is 123.4 kcal/mol. The result is C 2 rather than D 3 symmetry.…”

Section: More Of the [18]annulene Storymentioning

confidence: 99%

Crystal Structure Calculations

Allinger¹

2010

Molecular Structure

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After a force fi eld has been developed that is suitable for the calculation of molecular structures, it is fairly straightforward to extend it to cover assemblages of molecules, be they in the gas, liquid, or crystalline phase. We will start with the simplest cases fi rst. If we just put two acetone molecules together in the gas phase, with the general planes of the molecules parallel, so that the carbonyl groups are more or less one on top of the other and pointing in opposite directions, a molecular mechanics (or quantum mechanics) calculation will show that the two molecules are held together at substantially less than the van der Waals distances. The individual nonbonded interactions between the two molecules in MM4 calculations are the same (dipole -dipole and van der Waals) as those that would occur intramolecularly. The fact that they are between two different molecules really makes no difference. So if one wants to study gas -phase assemblages of molecules with MM4, in principle one does not have to do anything differently from the way it is done with a study of a single molecule, other than put the molecules together, and then optimize the total structure by minimizing the energy in the usual way. In practice, it may not be quite that simple.For even small assemblages of molecules, such as dimers and trimers, the multiple minimum problem may arise. That is, there may be different ways that one can put the molecules together and obtain minimum energy structures. One will probably want to

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The Stabilization Energy of [18] Annulene. A thermochemical determination

Cited by 22 publications

References 13 publications

Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

The Photoelectron Spectrum of [18]Annulene

Crystal Structure Calculations

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