Antiaromatic destabilization energies estimation

The estimation of aromatic stabilization (antiaromatic destabilization) energy based on thermodynamic characteristics of different reactions may yield for the same compound quite dissimilar values. As has already been pointed out, these discrepancies stem from the fact that the cyclic electron... 

Electrochemical evidence for the antiaromaticity of cyclobutadiene has been provided by Breslow and co-workers 17 The oxidation potentials for the hydro-quinone dianion 7 (—1.50 V, —0.68 V versus Ag — AgCl, at Pt electrode) are substantially more negative than the oxidation potentials of model 9 (—1.22 V, —0.45 V). In the 7 -8 conversion a dimethylenecyclobutene derivative, with only a small degree of possible cyclobutadiene character, is converted into a full cyclobutadiene, presumably partially stabilized by the ketofunctions. The data indicate that the cyclobutadiene resonance destabilization amounts to at least 12 kcal/mole, and an estimate of the true antiaromatic destabilization energy of 15—20 kcal/mole has been made17). 

At present, then, aromaticity is best defined in terms of stability derived from the delocalization of bonding electrons. An aromatic molecule is characterized by appreciable stabilization relative to a noncyclic polyene. An antiaromatic molecule is one that is destabilized relative to a polyene model, and the term nonaromatic can be applied to molecules for which the calculated energy and energy of the polyene model are comparable. Cyclobutadiene, with an estimated destabilization energy of 15-20 kcal/mol, is a good example of an antiaromatic species. 

Calculations using the complete basis set ab initio method for the cyclopropenyl radical give an ionization energy of 6.17 eV, in good agreement with an experimental energy of 6.60 eV, and an electron affinity of 0.45 eV. The very low value of the former is indicative of the large aromatic stabilization of the cation, and the low value of the latter indicates the instability of the cyclopropenyl anion. The radical is intermediate between the two, but these results do not permit an estimate of any antiaromatic destabilization of the radical. 

The resonance stabilization of benzene is commonly estimated to be about 36 kcal/mol (151 kJ/mol), but this is not an experimentally verifiable number. The 4n pi electron systems (4, 8,12, 16,...), with half-filled energy levels, not only are less stable than the 4n + 2 systems but are destabilized relative to their open-chain analog and are called antiaromatic. 

---