Antiaromaticity definition

Ronald Breslow and his collaborators have given some attention to the problem of estimating the degree of destabilization of cyclobutadiene with respect to nonconjugated models. They have concluded from electrochemical measurements of oxidation-reduction potentials of the system 37 38, of which only the quinone 38 has the cyclobutadiene fragment, that the C4H4 ring is destabilized by some 12-16 kcal mole-1 and so is definitely antiaromatic.15... 

Jerusalem Symposia on Quantum Chemistry and Biochemistry. Aromaticity, Pseudoaromaticity, Antiaromaticity, Israel Academy of Sciences and Humanities 1971 Vol. 3 E. Heilbronner On Mr Binsch s slide the following definition is proposed A conjugated electron system is called aromatic if it shows neither strong first-order nor second-order double bond fixation . Now, could you point out a molecule, except benzene, which classifies as aromatic G. Binsch Benzene is a perfect example E. Heilbronner Name a second one. 

Note There is another explanation relating to the definitions of aromatic and antiaromatic ring systems. See if you can explain the answer to this problem using these definitions. 

Several aspects of aromaticity have been studied <2002JOC1333> using statistical analyses of quantitative definitions of aromaticity. ASEs, REs, magnetic susceptibility exaltation (A), nucleus-independent chemical shift (NIGS), the harmonic oscillator model of aromaticity (HOMA), (/j) and (Aj), evaluated for a set of 75 five-membered 7t-electron systems and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations between the various aromaticity criteria, provided the whole set of compounds is used. The data in Table 9 have been found for arsole (AsH) 1 (E = As, R = H), its anion (As ), and protonated species (AsH2 ). 

Although we cannot as yet converge on a desired enthalpy of formation of gaseous [18]annulene, it is quite apparent that this last number is suspect in terms of at least two acyclic paradigms for aromaticity. Recall the Dewar-Breslow definition for aromaticity and antiaromaticity of an [njannulene in terms of the corresponding acyclic polyene with n/2 double bonds. There is no experimental measurement of the enthalpy of formation of all-( )-l,3,5,7,9,ll,13,15,17-octadecanonaene, species 151. However, we should be surprised if this value seriously differed from that of nine ethylenes and 8(5) kJmol , the 5 kJ mol being taken as the enthalpy of reaction 19 for unstrained olefins and dienes . The enthalpy of formation of 151 is thus ca 513 kJmol . This is somewhat less than the value for [18]annulene and so we would conclude that the cyclic species is essentially nonaromatic . Alternatively, consider the series of acyclic polyenes, ethylene, 1,3-butadiene, 1,3,5-hexatriene, The gas phase enthalpies of formation are respec-... 

This definition of resonance energy makes a clear distinction between aromatic (positive DRE), antiaromatic (negative DRE) and nonaromatic (near zero DRE) conjugated molecules. Extensive tables of resonance energies were also obtained in the framework of the HMO approximation by Hess and Schaad [Hess Jr. and Schaad, 1971a Hess Jr. and Schaad, 1971b Hess Jr. et al, 1972 Hess Jr. and Schaad, 1973]. Moreover, extensions and modifications of the calculations of the reference structure energy were proposed by other authors [Baird, 1969 Baird, 1971]. 

The described definitions have been widely used to characterize aromaticity. However, the correlation between these criteria and aromatic (or antiaromatic) nature is by no means simple. Even more so, the aromatic character is, by definition, a relative property, namely, one can assign aromaticity only by referring to model compounds. Thus, even when the number of 71-electrons calls for the existence of aromatic character, its extent cannot be predicted. Similarly the relationship between magnetic susceptibility of a polycycle and its conjugative stabilization is highly complex 105,106). 

The aceheptylene 23 is a system of 14 carbon atoms of which 13 carbons compose its conjugated periphery. Accordingly, 23 can be considered as a perturbed [13] an-nulene or as a (4n + 1 )tc conjugated system (n = 3) with an inner carbon atom. This system seems most suitable to examine the basic problem of patterns of delocalization in polycyclic anions. In view of the mentioned Platt s peripheral definition 87) such a compound is expected to be a noparomatic polyvinylic system, not exhibiting any aromatic or antiaromatic properties. This expectation is unambiguously confirmed. 

In terms of observable phenomena, the most useful definition of aromaticity is based on the magnetic anisotropy (10, 16-18) of a system as deduced from the XH NMR parameters. Antiaromatic 4n tt systems are expected to exhibit paratropic (high-field) shifts in their NMR spectra (11, 19) in contrast to the diatropic (low-field) shifts of (4n + 2) tt systems. Although the diatropic shifts revealed by (4n + 2) it systems are well-established, the experimental support for the relationship between para-tropicity and antiaromaticity is far less satisfactory (11). 

Now we would like to use a transition state ring bond order uniformity (n-molecular orbital delocalization) as a measure of its stability, and therefore the selectivity between two or more isometric transition state structures. A view that transition state structures can be classified as aromatic and antiaromatic is widely accepted in organic chemistry [54], A stabilized aromatic transition state will lead to a lower activation barrier. Also, it can be said that a more uniform bond order transition state will have lower activation barriers and will be allowed. An ideal uniform bond order transition state structure for a six-membered transition state structure is presented in Scheme 4. According to this definition, a six-electron transition state can be defined through a bond order distribution with an average bond order X. Less deviation from these ideally distributed bond orders is present in a transition state which is more stable. Therefore, it is energetically preferred over the other transition state structures. 

From MO theory can come a new definition of aromaticity In a cyclic, fully unsaturated system, if there are no unfilled bonding molecular orbitals, the system is aromatic, but if these MOs are only partly filled, the system is antiaromatic. We will discuss in section 6.3.4 that this concept also applies to heterocycles. 

The 3-cyclopropenyl anion is more basic than the allyl anion and cyclopropyl anion, its acyclic and saturated counterparts. This can be accounted for by the small central C-C—C bond angle and the resulting electrostatic repulsion in the constrained anion. No additional parameter is needed to account for the weak acidity of cyclopropene at the aUyhc position. Consequently, on the basis of the thermodynamic definition of antiaromaticity, this concept is not needed to describe the 3-cyclopropenyl anion. Magnetic criteria such as nuclear independent chemical shifts (NICSs) lead to a different conclusion, but in this instance there is no energetic basis for this view. Consequently, the 3-cyclopropenyl anion is best described as non-aromatic despite 50 years of thought to the contrary. 

The focus of this study is to find out to what extent different exocyclic (C=X) substituents induce aromaticity and antiaromaticity in cyclopropene derivatives (HC)2C=X [32]. Although the consensus opinion suggests that parent methylenecy-clopropene, (HC)2C=CH2, the simplest cross-conjugated cyclic hydrocarbon, is nonaromatic, it is admitted that cyclopropenone is at least modesdy aromatic [33-35]. However, quantitative assessments of the aromaticity of other (HC)2C=X derivatives have not led to satisfactory agreement. Thus, this family of compounds represents a paramount opportunity to use the EDA-aromaticity method to give a definitive answer to a question that has been controversially discussed in the literature for several decades [33-35]. 

All (4A7 -I- 2) annulenes clearly exhibit aromatic character, whereas 4 A7 annulenes are definitely antiaromatic, with negative HOMA. 

This approach allows one to calculate indirectly the heat of formation of C(sp )-C(sp ) single bonds, which is difficult to determine directly. With this approach, the definition of aromaticity is that an aromatic compound has DRE >0, an antiaromatic compound has a DRE < 0, and a compoimd that is neither aromatic nor antiaromatic has a DRE near 0. Results of DRE calculations by Baird are listed in Table 4.2. ... 

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