New aspects of the chemistry of nonbenzenoid polycyclic conjugated π-electron systems

Hafner, K.

doi:10.1351/pac198254050939

Cited by 70 publications

(40 citation statements)

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“…Stabilization of the structure without bond alternation (1B) and protection from reagents by bulky tert-butyl groups made possible the synthesis of 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI).S The chemistry of this and other polycyclic antiaromatics has been reviewed in recent years. 6 According to X-ray analysis of the crystal s t r~c t u r e ,~ the TTBI molecule has C-C ring bond lengths more balanced than those corresponding to (1 A), suggesting delocalization of A electron density in agreement with theoretical expectations. A quantitative estimate of the structural parameters of TTBI and its parent molecule, s-indacene, is absent in past reports.…”

Section: Introductionmentioning

confidence: 54%

The ground state of antiaromatic 1,3,5,7-tetra-tert-butyl-s-indacene

Gellini¹,

Cardini²,

Salvi³

et al. 1993

J. Phys. Chem.

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The electronic and vibrational properties of the antiaromatic molecule 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI) in the ground state are investigated in this paper. MO ab initio calculations have been performed on TTBI and the parent molecule, s-indacene (I). The results show that the C-C bonds of s-indacene alternate in length and the corresponding C2h structure is more stable than the structure without bond alternation (D2h symmetry), regardless of the basis set used. Bond length alternation is decreased by alkyl substitution in the 1, 3, 5, and 7 positions. The optimized TTBI structure calculated with the 3-21G basis set is in good agreement with X-ray diffraction data on the molecule. The torsional motion of one butyl group with respect to the indacene ring gives rise to two stable conformers. Experimentally, ground-state properties have been studied by means of infrared and Raman spectroscopy. At low temperature both spectra show narrow bands. Using 568.2-nm excitation the Raman spectrum is enhanced by resonance effects due to the proximity of the strongly active SZ state and, as a consequence, a large number of Raman modes are observed. The vibrational assignment is discussed by means of semiempirical and ab initio calculations of frequencies and intensities of model molecules such as I, 1 -tert-butyl-indacene (TBI) and 1,5-di-tert-butyl-indacene (DTBI). The infrared spectrum is in remarkably good agreement with predictions from semiempirical calculations. Comparison with calculated frequencies also allows the assignment of the most intense Raman lines.

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Section: Introductionmentioning

confidence: 54%

The ground state of antiaromatic 1,3,5,7-tetra-tert-butyl-s-indacene

Gellini¹,

Cardini²,

Salvi³

et al. 1993

J. Phys. Chem.

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“…Such a change of the nature of the ground state reduces considerably the energy difference between the energy-minimum state (GI,) and the saddle point (DZl,) (21). In fact, all the energy differences so far calculated by assuming the ground state at the saddle point to be the closed-shell state (9-11, 14, 15) are too large to be comparable to the experimental activation energies for the isodynamic double-bond shift isomerization reaction in pentalene and heptalene (19,20).…”

Section: Pentalene and Heptalenementioning

confidence: 96%

“…It is thus expected that in such molecules the exchange integral responsible for the energy separation between the excited singlet and triplet states originating from the electron jump HOMO + LUMO is rather small, and the SP effects lead to the violation of Hund's multiplicity rule. Now it has well been known both theoretically (9-11, 14, 15) and experimentally (16)(17)(18)(19)(20) that the ground states of pentalene and heptalene undergo the molecular-symmetry reduction DZh + CZh, as that of propalene does. The geometrical structures with respect to the C-C bond lengths of the symmetrical (D,,) and distorted (Cz,,) structures optimized by using the MNDO method are shown in Fig.…”

Section: Pentalene and Heptalenementioning

confidence: 99%

Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons

Koseki¹,

Nakajima²,

Toyota³

1985

Can. J. Chem.

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This paper is dedicated to Professor Camille Sandor-on the occasiotz of his 65th birthday SHIRO KOSEKI, TAKESHI NAKAJIMA, and AZUMAO TOYOTA. Can. J. Chem. 63, 1572Chem. 63, (1985. Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons is studied by using the Pariser-Pam-Pople type SCF MO method and the ab initio MO method with STO-3G basis set, both methods being augmented by CI-type treatments. It is shown that for symmetrical structures (Dzl,) of the nonalternant hydrocarbons, propalene, pentalene, and heptalene, the lowest excited singlet state is energetically lower than the corresponding triplet state. This is mainly due to the spin polarization (SP) effects. For D2/, structures of pentalene and heptalene the open-shell excited singlet state is predicted to be lower in energy than the closed-shell state, with the result that the former is really the ground state. Further, calculations made by including electron correlation effects reveal that in linear polyenes and polyacenes, the lowest excited singlet "minus" state (using Pariser's classification of the alternancy symmetry species) is lower in energy than the corresponding triplet state. The energy lowering of the singlet "minus" state in linear polyenes is due mostly to the mixing with the doubly excited configurations (tnm + nn), while the considerable part of it in polyacenes is due to the SP effects. [Traduit par le journal]

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“…Observed (ll31,15 K) and calculated (fundamental, overtone and combination a, modes, 1-tert-butyl-s-indacene) FranckCondon intensities of the SO -s g transition. Calculations (vertical lines) include Duschinsky mixing in the excited state.…”

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confidence: 99%

Excited states of s-indacene

Gellini¹,

Angeloni²,

Salvi³

et al. 1995

J. Phys. Chem.

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The electronic properties of 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI), the stable derivative of antiaromatic s-indacene, have been studied via absorption and resonance Raman spectroscopy at low temperature. The two strongest absorption regions, at e550 and 320 nm, show Franck-Condon structures suggesting change of excited-state geometry with respect to SO. Large enhancement factors have been measured for the most active Raman modes when the excitation frequency goes through the S2 vibronic structure. By means of semiempirical QCFF-PI calculations ground-and excited-state optimized geometries and vibrational frequencies have been obtained for a quantitative estimate of vibronic Franck-Condon activities in the S2 and s 6 states. The effect of normal-mode rotation in the excited state (Duschinsky effect) and alkyl substitution on the electronic and resonance Raman spectra of TTBI is discussed.

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New aspects of the chemistry of nonbenzenoid polycyclic conjugated π-electron systems

Abstract: Abstract

Cited by 70 publications

References 0 publications

The ground state of antiaromatic 1,3,5,7-tetra-tert-butyl-s-indacene

The ground state of antiaromatic 1,3,5,7-tetra-tert-butyl-s-indacene

Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons

Excited states of s-indacene

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