[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]

The symmetry of the first form is the first form itself.

My bad.

[Enthalpy]

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

I don't grab this logic. Does Hückel's rule tell anything more than "aromatic"? In 1,2-dichlorobenzene, I don't expect both forms to be equally probable.

[njoyeux]

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

I don't grab this logic. Does Hückel's rule tell anything more than "aromatic"? In 1,2-dichlorobenzene, I don't expect both forms to be equally probable.

I don't grab the logic either.

Plus, on the following research paper pyracylene is described as a bordeline case of aromaticity.
https://pubs.acs.org/doi/pdf/10.1021/ja012541%2B


What does that tell us?