Paratropic Delocalized Ring Currents in Flattened Cyclooctatetraene Systems with Bond Alternation

Fowler, Patrick W.; Havenith, Remco W. A.; Jenneskens, Leonardus W.; Soncini, Alessandro; Steiner, Erich

doi:10.1002/1521-3773(20020503)41:9<1558::aid-anie1558>3.0.co;2-g

Cited by 61 publications

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References 29 publications

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“…In the tub-shaped equilibrium geometry, COT has lost its paratropic ring current, but a minor distortion, entailing little increase in energy, would be sufficient to restore it. Indeed, the 1 H NMR spectra of the recently synthesized nonplanar cycloprop [8]annulene 12e support this view; CTOCD-DZ/6-31G**//RHF/6-31G** calculations show that this molecule sustains a paratropic ring current ( Figure 5). Figure 5.…”

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confidence: 85%

“…RHF/6-31G** (B3LYP/6-31G**) 127.3°8 (127.7°)). Support for this view is given by the very small differences between the RHF/ † Theoretical Chemistry Group, Utrecht University (affiliated with Organic Chemistry and Catalysis 6-31G** optimized bond lengths 8 in D 4h (∆R ) 0.153 Å) and D 2d geometries (∆R ) 0.155 Å; expt. 6,7 ∆R ) 0.14 Å) (For B3LYP/6-31G**, see Supporting Information, Table S24).…”

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confidence: 99%

“…Figure 5. CTOCD-DZ/6-31G**//RHF/6-31G** π map of cycloprop- [8]annulene 12e plotted at 1a 0 above the median plane. The RHF/ 6-31G** optimized geometry has C 2 symmetry with the axis shown horizontal.…”

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confidence: 99%

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Persistence of Paratropic Ring Currents in Nonplanar, Tub-Shaped Geometries of 1,3,5,7-Cyclooctatetraene

2006

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Surprisingly, the anti-aromatic paratropic ring current of the planar D 4h transition state of 1,3,5,7-cyclooctatetraene (COT) survives almost all the way to the equilibrium tub-shaped D 2d COT structure (plane-to-plane distance, d ) 0.76 Å); the current vanishes at ca. 80% of the geometric change and ca. 90% of the energy change from planar to tub geometry. 1,3,5,7-Cyclooctatetraene (COT) is a paradigm anti-aromatic 4n π electron system. In an equilateral planar D 8h geometry, COT is predicted by simple Hückel theory to have a triplet ground state, but the real ground state has a closed-shell singlet electronic configuration and a nonplanar tub-shaped equilibrium geometry of D 2d symmetry. 1 On the original definition of an anti-aromatic system as a "cyclic conjugated system [that] has its π energy higher than that of a suitable reference compound, which is not cyclically delocalized", 2 it is reasonable to suppose that the inherent anti-aromaticity of the 8π electron cycle contributes to the nonplanarity of the equilibrium structure. However, calculations suggest that the π resonance energy is negligible in planar conformations of COT 3 and the larger [4n]-annulenes, 4 indicating that the π electrons have no role in driving COT to nonplanarity. An interpretation consistent with ideas of both strain release 4,5 and π-distortivity 6 would be that the COT system moves from the equilateral D 8h to the bond-alternated D 4h structure by a first-order Jahn-Teller (π-electronic) distortion and that further distortion to the out-of-plane D 2d geometry is driven primarily by release of angle strain, as C-C-C bond angles are thereby reduced from the 135°angle of the planar octagon toward the ideal sp 2 angle of 120°. From a combination of gas-phase electron-diffraction and single-crystal X-ray mea-

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Persistence of Paratropic Ring Currents in Nonplanar, Tub-Shaped Geometries of 1,3,5,7-Cyclooctatetraene

2006

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“…Thus geometry may be a source of reliable information on the electron distribution, and employing appropriate references, may be used for description of π-electron delocalization. Thus aromaticity index HOMA [6,7] may be used firmly [11] for estimation of π-electron delocalisation except in a few cases like planarized cyclooctatetraene [12] and alike [13]. The extended form of HOMA [7] allows one to get information about the nature of dearomatisation.…”

Section: Methodsmentioning

confidence: 99%

How the Substituent Effect Influences π-Electron Delocalisation in the Ring of Reactants in the Reaction Defining the Hammett Substituent Constants σm and σp

Krygowski

Stępień

Cyrański

2005

IJMS

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Application of the geometry (HOMA, EN, GEO) and magnetism based (NICS, NICS(1)) indices of aromaticity to optimised geometry of the ring in 12 meta -and 12 para -substituted benzoic acids and their anions by use of DFT computations at B3LYP/6-311+G(d,p) level has shown a very low substituent effect on the π-electron delocalisation. This resembles (qualitatively) the resistance of benzene (and typical aromatic systems) against reactions leading to the change of π-electron delocalisation.

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“…Magnetic properties are evaluated for benzene-like structures at a geometry where the inner benzene-moiety has a delocalised p-system (3b), and at a geometry where the p-system is localised (3a, Chart 2). [10][11][12][13] Chart 1 The valence bond wavefunction (C) written as a superposition of a covalent structure A and the two possible ionic structures B and C. The coefficients c i are variationally optimised in a VB calculation.…”

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confidence: 99%

On the interpretation of valence bond wavefunctions

et al. 2007

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Valence bond wavefunctions are naturally geared to the chemist's idea of chemical bonding. In a structure one may distinguish different electron pair bonds and possible radical character. A structure may correspond to a covalent bond, where all electrons are equally divided over the atoms, or may describe an excess charge on a discrete part of the molecule, which indicates ionic bond character. From the weights of the structure in a variationally optimised multi-structure valence bond wavefunction one may derive the importance of the different bonding types. The individual structures could be considered to represent the different physical situations. We explore this concept for simple diatomic molecules and for polyatomics, and we discuss the relation to Lewis structures. We show that the assumption of individual properties for the individual structures leads to inconsistencies.

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Paratropic Delocalized Ring Currents in Flattened Cyclooctatetraene Systems with Bond Alternation

Cited by 61 publications

References 29 publications

Persistence of Paratropic Ring Currents in Nonplanar, Tub-Shaped Geometries of 1,3,5,7-Cyclooctatetraene

Persistence of Paratropic Ring Currents in Nonplanar, Tub-Shaped Geometries of 1,3,5,7-Cyclooctatetraene

How the Substituent Effect Influences π-Electron Delocalisation in the Ring of Reactants in the Reaction Defining the Hammett Substituent Constants σm and σp

On the interpretation of valence bond wavefunctions

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