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Resonance forms of pyracyclene

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njoyeux:
Hello all,

I have a question regarding the resonance of pyracyclene.


If I’m correct, pyracyclene has four resonance forms:








Now my problem is estimating the dominance of each form. Are they equal or is there one which is more dominant?

This is what I’ve got so far:

Rules for estimating stability of resonance structures

- The greater the number of covalent bonds, the greater the stability since more atoms will have complete octets.
- The structure with the least number of formal charges is more stable.
- The structure with the least separation of formal charge is more stable.
- A structure with a negative charge on the more electronegative atom will be more stable.
- Positive charges on the least electronegative atom (most electropositive) is more stable.
- Resonance forms that are equivalent have no difference in stability and contribute equally (eg. Benzene).

Pyracyclene has the same carbon atoms, the same number of covalent bonds and a formal charge of zero. I don’t see any difference in stability either. So it seems that all forms are equivalent.

Is this correct?

Enthalpy:
The second and third forms are symmetric up/down, but the first form has no such symmetric form listed. Did you possibly miss it?

njoyeux:
No, I didn't miss it. The symmetry of the first form is the first form itself. Don't be fooled by the double bond in the middle. This bond is symmetric up/down (I know, the double bond symbol is confusing).

The pyracyclene molecule has two symmetries: up/down and left/right. You can see that all symmetries for all forms are present.

Orcio_87:
It looks like it fullfils the Huckel's rule, so all four structures should be equal.

Enthalpy:

--- Quote from: njoyeux on November 08, 2022, 08:19:43 AM ---The symmetry of the first form is the first form itself.
--- End quote ---
My bad.

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