Antiaromaticity energetics

As the collection of recent reviews in the topic shows [12-20], a rather large consensus appears in the computation or experimental tests for the diagnosis of the aromaticity/antiaromaticity (energetic, structural, magnetic, chemical reactivity, and electronic diagnostic tools), whereas the mechanisms themselves still remain open to the debate. The primary controversy in the area involves the questions of whether aromaticity/antiaromaticity can be quantified and, if so, which of the methods commonly used to evaluate aromaticity/antiaromaticity is most appropriate. The literature on aromaticity and its measure is so vast that I must be content here with outlining briefly only the aromaticity indicators which have been extensively used to diagnose aromaticity/antiaromaticity in the domain of all-metal aromatics. 

Oxidation of p- and m-substituted diphenylmethylidene fluorenes resulted in the formation of a suite of dications, 9 and 10, which were shown to be antiaromatic through magnetic and energetic criteria. ... 

The most common evaluation of aromaticity via energetic criteria is done using calculations either a type of isodesmic reaction (34) or comparison of two isomers that differ only through the aromaticity of one (3). We were interested in the possibility of evaluating stability experimentally and the electrochemical formation of dications such as 8 was attractive. In this approach, the redox potential for formation of the dication would be compared to the redox potential for formation of dications which could not be antiaromatic. If 8 was antiaromatic, its redox potential should be larger and more positive than that of the reference system. This approach was applied to the evaluation of the antiaromaticity of 9... 

While the redox potentials for formation of 9 and 10 indicates the greater instability of these dications compared to reference systems, is this instability due to antiaromaticity The plot of the redox potentials vs the sum of the NICS values for the fluorenyl system, Figure 2, next page, shows a reasonable relationship between these energetic and magnetic measures of antiaromaticity. 

In the majority of the systems examined, the magnetic and energetic criteria used gave similar results, suggesting that either is an effective measure of antiaromaticity in fluorenylidene dications. 

Table IV. Energetic and magnetic measures of antiaromaticity for fluorenyl systems, 7, Y=H, and 3, indenyl systems, 18 and 19, vide infra, and indenylidenfluorene dications... 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

Though it is usually not difficult to classify a given compound as aromatic, nonaromatic or antiaromatic from a qualitative point of view, much more complex problems arise in attempting to describe the aromaticity in quantitative terms. Until now, three main groups of quantitative criteria of aromaticity have been elaborated energetical, structural and magnetic. 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

The most likely factor is electronic since MM makes no reference to electrons, it should not be expected to reflect structural and energetic effects arising from, say, aromaticity and antiaromaticity, encapsulated in the 4n + 2 and the corollary 4n rules [1-3]. 

Cyclooctatetraene would be antiaromatic if Htickel s rule applied, so the conjugation of its double bonds is energetically unfavorable. Remember that Htickel s rule applies to a compound only if there is a continuous ring of overlapping p orbitals, usually in a planar system. Cyclooctatetraene is more flexible than cyclobutadiene, and it assumes a nonplanar tub conformation that avoids most of the overlap between adjacent pi bonds. Hiickel s rule simply does not apply. 

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