Antiaromatic destabilization energies

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

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). 

Antiaromatic Destabilization from the Energies of Isodesmic, Homodesmotic, and Hyperhomodesmotic Reactions... 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

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. 

The MNDO calculations on sila-, germa-, and stannacyclopentadienyli-denes have shown that whereas for cyclopentadienylidene (272) the energies of the antiaromatic 47t- and the aromatic 6ir-electron structures are close in value (89UK1067), in the (273)-(275) series the 67r-electron structures are quite noticeably destabilized (Table XXIII). Unlike (272), the electronic ground state of compounds (273)—(275) correspond to minima on the PES. These results point to the diminished role of antiaromatic destabilization in the 47r-electron structure (273)—(275), as opposed to (272). It should therefore be expected that these molecules would be more stable than (272). This has indeed been confirmed by our calculation on the heats of the isodesmic reaction (85) (Table XXIII). 

This additional energy content is attributed to an antiaromatic destabilization of the bicyclopentene ground state. The homoantiaromatic character of bicyclopentene has been postulated on the basis of quantum mechanical calculations (94) and has recently also been derived from the photoelectron spectrum (95). 

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. 

Dibenzoannelation of cyclobutadiene results in biphenylene (50), a long-known isolable hydrocarbon with an archival gas-phase enthalpy of 417.9 3.3 kj/mol. That its enthalpy of formation is essentially the same as the monobenzoannelated cyclobutadiene speaks to the antiaromaticity of the latter. An ab initio study of benzocyclobutadiene and biphenylene found that the intrinsic destabilization energy is ca. 33 kJ/mol greater in the former compound." This does not mean biphenylene is not destabilized relative to benzenoid expectations. The destabilization of biphenylene of more than 200 kJ/mol relative to triphenylene or biphenyl as explained earlier is huge, although not as large as for cyclobutadiene itself. The benzene rings clearly ameliorate the antiaromaticity of cyclobutadiene. 

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