MN-effect and some Common Fallacies

Differences in CC bond lengths of the aromatic moiety, which are less than 0.025 A are indeed small if compared to distances of 1.48 A and 1.33 A characterising C(sp2)-C(sp2) single and C(sp2)=C(sp2) double bonds, respectively, as usually pointed out by X-ray researchers. However, they are quite significant for many properties which critically depend on interatomic distances like e.g. molecular quadrupole moments, electric field gradients, spin-spin coupling constants of directly bonded atoms etc. A 

The fact of the matter is that MN-distortions of the aromatic nucleus are affected by rehybridization, 7r-electron conjugation and/or hyperconjugation. These components are intermingled and they are not always easily resolved and separated. We have been able to show that appearance of bent bonding did not invalidate arguments in favor of the MN-alternation of CC bonds in fused benzenes and their fluoroderivatives [21]. 

In conclusion, the moral of this story is that one should define very clearly relevant questions related to the phenomenon under study and then select adequate techniques and/or methods able to provide reliable answers. It is also of crucial 

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