Canonical structures pyrrole

In contrast to pyridine, the unshared pair in canonical structure A in pyrrole is needed for the aromatic sextet. This is why pyrrole is a much weaker base than pyridine. 

The dipole of pyrrole points in the direction 4.120. The explanation usually offered is that the orbitals are polarised in the sense illustrated by the canonical structures 4.121 and 4.122. This cannot be the reason, because this is only one of the n orbitals, the HOMO. We can see on the drawing 4.54 that the total n charge, derived from the coefficients on Fig. 1.54, is, as ought to have been expected, higher on the nitrogen atom and lower on the carbon atoms. The same must be true in the cr framework. Why then is the dipole pointing in the other direction ... 

Isoindole is a 7r-excessive 10-7r-electron heteroaromatic system. In valence bond terms the delocalized system can be described in a conventional way by the canonical forms 57-61, where 57 is of major importance. The situation here is rather analogous to that with pyrrole, but the dipolar structures... 

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