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To my knowledge, resonance happens where more than one lewis stucture can represent the compound with no shift in sigma bonds

so, suppose in a simple conjugated system even like 1-3 pentadiene , two forms are well known, i want to know if a third form where each atom has completely filled or completely empty p orbitals should also be taken into consideration, as technically, there is nothing wrong with it, I accept that it's "contribution" will be negligible, but it should be counted in all POSSIBLE resonating structures

https://commons.wikimedia.org/wiki/File:Naphthalene_resonance_structure.svg

similar argument here, a lot more can be created if we allow charge separation of pi bond

EDIT: The question is whether there is a convention of some sort which tells us which ones to count which ones not (if we cant tell the magnitude of difference in stability of the structure)

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  • $\begingroup$ These resonance structures are valid but they usually provide a very small contribution to the overall structure. $\endgroup$
    – bon
    Feb 1, 2016 at 21:41
  • $\begingroup$ I wrote that :) $\endgroup$
    – Mrigank
    Feb 1, 2016 at 21:42
  • $\begingroup$ So what is your question? What you have written is correct. $\endgroup$
    – bon
    Feb 1, 2016 at 21:45
  • $\begingroup$ in a question where it asks us to tell the total number of resonating structure, theoretically, not only the ones in majority, so is there a convention which limits certain kind of structure to not be included? $\endgroup$
    – Mrigank
    Feb 1, 2016 at 22:10
  • $\begingroup$ also, chemistry.stackexchange.com/questions/15665/… here, how a sigma bond can be broken in resonating structures? i thought that it was a necessary condition taht only pi electrons move $\endgroup$
    – Mrigank
    Feb 1, 2016 at 22:37

1 Answer 1

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Resonances structures are a model of the quantum mechanical orbital mixing. To my mind they are a complete farce. They are also quite useful

A model is a simplification of a problem, usually based on ignoring negligible factors. So this question really distills down to what is negligible and what isn't?

That depends on roughly two factors:

  1. The conditions of reaction.

Are the conditions strongly ionic or acidic? Then charge separation may become relevant.

  1. The rigor of the problem.

Is this for a high school course? A Ph.D. thesis? A bar-room chat with your mates?

Unfortunately, like most of chemistry, there is not an absolute answer here.

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