Aromaticity/antiaromaticity

The aromatic-antiaromatic transition state rules are. another formulation of the Woodward-Hoffmann type rules (Table 8.3). 

Aromaticity/antiaromaticity in cluster systems has certain peculiarities when compared with organic compounds. The striking feature of chemical bonding in cluster systems is the multifold nature of aromaticity, antiaromaticity, and conflicting aromaticity [3-10]. Double aromaticity (the simultaneous presence of [Pg.439]

In Table 29.1, we summarized our results of electronic structure calculations for Bf -Bjj. We reported symmetry, spectroscopic state, valence electronic configuration, number of 2c-2e peripheral B-B a-bonds, number of delocalized o- and TT-bonds, and assignment of global aromaticity/antiaromaticity. The description of chemical bonding in terms of 2c-2e peripheral B-B a-bonds and nc-2e delocalized a- and tt-bonds was obtained via AdNDP method at... 

Providing a Context for Antiaromaticity the Aromaticity/antiaromaticity Continuum... 

That is, just as oxidation of tetrabenzo[5.5]fulvalene resulted in formation of two antiaromatic fluorenyl cations linked by a single bond, reduction should give two aromatic fluorenyl anions, linked by a single bond, 20. We call this relationship the aromaticity/antiaromaticity continuum and can apply the same measures of aromaticity to 20 as were applied to 3, to evaluate relative aroma-... 

Estimation of the aromaticity (antiaromaticity) of various compounds from the values of the ISE, HSE, and HHSE will be discussed in their respective sections. For the present, we will merely observe that these values, some of which are given in Table II, correlate with those of the HSRE and TRE. 

Aromatic, antiaromatic, and acyclic polyenes primarily differ in bond lengths, which serves as a basis for the structural indices of aromaticity reflecting the degree of alternation of bond lengths in a ring. 

As has been shown by DRE (72JA4941) and TRE (78BCJ1788) calculations, aromatic (antiaromatic) character is often inverted in the lowest excited state. Therefore, for a molecule with an aromatic ground state one may expect antiaromatic destabilization of the lowest excited state and a sizeable energy gap between them. Conversely, for molecules with the antiaromatic ground state this gap will be much smaller. 

Depending on the type of the heteroatom (X, Y, or Z) that replaces the —CH= group in the original hydrocarbon, heterocyclic structures may be designed with either the same number of 7r-electrons as in the parent molecule, or with a lesser or greater number of these. In the latter case, a change in the number of the 7r-electrons must reverse the aromatic (antiaromatic) character relative to the hydrocarbon analog. 

Clearly, in view of a diversity in the types of heterocyclic compounds, one may hardly expect that all the manifestations of their aromaticity (antiaromaticity) could be rationalized in terms of some simple regularities. We shall therefore attempt to trace certain characteristic trends in the dependence of the aromaticity on the type of heteroatoms, their number and positions in the molecular structure. Our reasoning will be based on the nature of the aromaticity criteria and of the electron count rules. Then turning to individual compounds, we shall add details to the picture. 

According to structural criteria, in particular, to the SINDOl-calculated (90JPC4467) values of RCI, the excited states of pyridine, as distinct from the ground state, do not possess aromatic character. This finding confirms the aromaticity antiaromaticity (nonaromaticity) inversion in the excited state relative to the ground state. 

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... 

From an aromaticity-antiaromaticity point of view, an intriguing representative of the four-membered heterocycles is undoubtedly the hitherto unknown azete (21 R1=R2 = R3 = H). Theory... 

The importance of the arguments we have outlined lies in the fact that they provide a theoretical foundation both for aromaticity-antiaromaticity and for pericyclic selection rules. They furthermore demonstrate the relationship between the two The topological equivalence between an array of p orbitals in a w system of a carbon chain or ring and a pericyclic transition state, composed of an... 

Avital Shurki was born in 1970 after two years in the army, she began studying chemistry in 1990 at The Hebrew University, where she finished summa cum laude and was on the Dean s and Rector s Lists. In 1995 she received the Sara Wolf Prize for Graduate students. In 1994 she joined the group of S. Shaik and completed her Ph.D. degree in Quantum Chemistry in 1999. One of the main themes in her dissertation was the aromaticity, antiaromaticity, and -delocalization. In 2000 she received the Rothchild Fellowship and joined the group of Professor A. Warshel at USC. Her research interests involve applications of VB ideas and calculations to chemical reactivity and bonding. 

Chem Rev 105(10) whole issue devoted to aromaticity, antiaromaticity and related topics... 

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