Geometry, antiaromatic

The potential surfaces of the ground and excited states in the vicinity of the conical intersection were calculated point by point, along the trajectory leading from the antiaromatic transition state to the benzene and H2 products. In this calculation, the HH distance was varied, and all other coordinates were optimized to obtain the minimum energy of the system in the excited electronic state ( Ai). The energy of the ground state was calculated at the geometry optimized for the excited state. In the calculation of the conical intersection... 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

Pi Nonbonded Interactions and Pi Aromatic, Nonaromatic, and Antiaromatic Geometries... 

This pattern is familiar to the theoretical organic chemist since it reflects nothing more than the fact that the B + C union in the all-c/ s geometry resembles an antiaromatic structure while the same union in the all-tarns geometry resembles a non-aromatic structure. We have already encountered such situations in our discussion of the conformational isomerism of 1,3-butadiene. 

The molecule C16H16 is formally Huckel antiaromatic in a planar geometry. However, appropriate structural modifications may selectively accentuate the aromatic unions and lead to a Ci6H16 molecule which is not antiaromatic. 

Structural indices constructed in this fashion are, in essence, phenomenological, and one is entitled to ask whether the specific features in the geometry of the aromatic and antiaromatic molecules are indeed determined, and if so, to what degree, by the cyclic electron (bond) delocalization. 

Distinguishing Characteristics in the Geometry of Aromatic and Antiaromatic Molecules... 

Before turning to manifestations of aromaticity or antiaromaticity, relative stability, geometry, and other characteristics of the molecules represented in Scheme 3, we wish to treat in some detail certain regularities in changes of aromaticity depending on the type of heteroatom as well as the relationship between the geometry of a given molecule and its aromatic (or antiaromatic) character. 

Aromatic cyclic 7r-electron delocalization does indeed stabilize the planar structure with bond equalization (84ZOR897)—the problem is that, in addition to that effect, there may exist some others that may eventually overshadow it. Thus, the foregoing warrants the conclusion that the preference of a planar or nonplanar geometry of heterocycle depends on a number of factors including aromaticity (antiaromaticity), which may not even be the most important. In any case, this factor should not be disregarded if one wishes to obtain a correct overall energy balance. For example, aromaticity is reflected in the values of inversion barriers. Thus, for antiaromatic 2-azirine the nitrogen inversion barrier is, as was mentioned earlier, 37.7 kcal/mol, whereas in the case of its saturated... 

Modified intermediate neglect of differential overlap (MINDO/3) and modified neglect of differential overlap (MNDO) methods with full geometry optimization calculate triazol[4,5-, [l,2,3]triazole to possess antiaromatic structure with C h symmetry but not 1)2,5. In contrast, the aromatic structure with 1)2,5 symmetry is found advantageous for the 2,5-dioxide derivative <1991IZV1825>. 

X and Y -CH=CH-). This suggests that the geometry distortion around the C-F bonds in [27](X and Y CH=CH-) when compared with the [27](X = Y = H) geometry, increases the corresponding As values between the fluorine lone pair orbitals and the (C-F) antibonding orbitals. It should be recalled that the d(F-F) distance in [27](X and Y -CH=CH-) is notably larger than in [27](X = Y = H). There is also about 2 Hz difference between the C-C contributions. Probably, this difference comes from the difference in aromaticity between these two compounds in fact while [27](X = Y = H) is an aromatic compound, [27](X and Y CH=CH-) is an antiaromatic compound. This seems to indicate that the outlier condition of [27]... 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

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