Is delocalization a driving force in chemistry? Benzene, allyl radical, cyclobutadiene, and their isoelectronic species

Abstract:The title journey is undertaken at the levels of both theory and experiment. Since 1983, homoaromaticity has been shown to play at most a minor role in the stability of Scott's [N]pericyclyne hydrocarbons-the first ring carbo-mers of cycloalkanes. This statement has been systematically refined for N = 3-6 by using both classical theoretical tools and newly designed tools based on electron localization function (ELF) analysis. The compatibility of the [5]-and [6]-pericyclyne cores with vertex functionalities was established by the synthesis of 20 oxy (carbo-cyclitol) derivatives. The stereoisomeric resolution of two of them has been achieved. pericyclynes are actually potential precursors of the carbo-benzenes. Criteria based on density functional theory (DFT) calculations (magnetic, energetic, structural/"electronic") show that the aromaticity of carbo- [N]annulenic species is comparable to that of their parent molecule. This has been challenged by the synthesis of several novel carbo-benzenic molecules with various substitution patterns. The theory-experiment interplay is pursued by considering ring carbo-mers of other conjugated ring systems such as radialenes. The second carbo-mers (butadiyndiyl-expanded rings) of [3]radialene and benzene are also envisioned. Hexaphenyl-carbo 2 -benzene has been observed by 1 H NMR and UV-vis spectroscopy.

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“…In accordance with Shaik's theory [31], the isogyric ASEs of Fig. 11 explicitly refer to both the π-and σ-electron systems.…”

supporting

confidence: 66%

Ring carbo-mers: From questionable homoaromaticity to bench aromaticity

Zou

Lepetit

Coppel

et al. 2006

Pure and Applied Chemistry

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“…Restoring the balance by decreasing the strengths of both and bonds returns the geometry back to an asymmetrical structure. This suggests that the system is an important factor in the determination of the geometry, which can be understood in terms of the model given by Shaik et al [17][18][19][20][21][22][23]. Zilberg and Haas [25] stated that the geometric distortion of cyclobutadiene is a fundamental property of 4n electron ring systems.…”

Section: Discussionmentioning

confidence: 99%

“…Originally, the symmetrical structure of benzene, which was always considered a key feature of aromaticity, was considered just an effect of the interacting electrons. Shaik et al [17][18][19][20][21][22][23] challenged that view and stated that the system of benzene favors a distorted geometry with localized bonds. The system forces the molecule to be symmetrical.…”

Section: Introductionmentioning

confidence: 99%

Valence bond descriptions of benzene and cyclobutadiene and their counterparts with localized bonds

Dijkstra

Lenthe

Havenith

et al. 2003

Int J of Quantum Chemistry

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Geometry optimizations have been performed for benzene and cyclobutadiene and for the corresponding moieties with nonresonating double bonds, viz. 1,3,5-cyclohexatriene and 1,3-cyclobutadiene. The calculations were done using the valence bond self-consistent field method including orbital optimization. Both strictly local and delocalized p-like orbitals were used for the system, which influences the strengths of the bonds. The calculations result in geometries and resonance and stabilization energies for benzene and cyclobutadiene, which are compared with theoretical models of aromaticity. The importance of resonance is discussed.

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“…Due to second order Jahn-Teller effect [42], electronic states close to each other may interact and for n sufficiently large a structure with bond alternation may be energetically advantageous even for the higher annulenes satisfying the Hückel rule [43,44]. Following early recognitions of the propensity of π electrons to distortivity [18,45], the rather surprising statement was advanced in a series of important papers [17,[46][47][48][49][50][51][52][53]. The delocalized structure of benzene was seen as intermediate configuration between the localized K A and K B structures in the bond exchange process K A → K B along the distortion mode where CC stretchings alternate with CC shrinkings.…”

Section: π-Electron Approximationmentioning

confidence: 99%

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Gellini

Salvi

2010

Symmetry

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The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]-and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

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Is delocalization a driving force in chemistry? Benzene, allyl radical, cyclobutadiene, and their isoelectronic species

Cited by 242 publications

References 14 publications

Ring carbo-mers: From questionable homoaromaticity to bench aromaticity

Ring carbo-mers: From questionable homoaromaticity to bench aromaticity

Valence bond descriptions of benzene and cyclobutadiene and their counterparts with localized bonds

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

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