Azulene charge distribution

In keeping with its aromatic character and unsymmetrical charge distribution, azulene undergoes certain typical electrophilic substitution reactions at the 1 and 3 positions. Thus Friedel-Crafts acylation leads to a mixture of 1-ethanoylazulene and 1,3-diethanoylazulene ... 

Charge distributions and bond orders of the cation and anion of 2//-benz[crf]azulene, on the other hand, are dissimilar. The seven-membered ring of the cation is delocalized, containing most of the positive charge, while that of the anion 87 is localized and the negative charge resides mainly in the delocalized indenyl moiety. A delocalized indolizine part with a localized butadiene bridge in the isoelectronic cycl-... 

One of the deficiencies of the MO methods, especially the simple ones, is that they tend to exaggerate uneven distribution of electrons in a molecule and thus make it more polar (with a higher dipole moment) than it actually is. The result is that dipole moments which are based on charge densities obtained from eigenfunctions of the MO approximations are usually considerably higher than the actual experimental dipole moments. Two old, well-known examples of theoretical dipole moments obtained by the HMO method are fiilvene (11) whose calculated dipole moment is 4.7 D [72-74] and the experimental value is 1.2 D [68], and azulene (15), with a calculated dipole moment of 6.9 D [72] and the experimental value of 1.0 D [68]. 

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