Comments on: Frost Circles

By: patrizio cecchi

patrizio cecchi — Mon, 30 Sep 2024 10:19:14 +0000

To James Ashenhurst. The Huckel rule and the derived Frost circle are constructed on energy grouds, i.e. on thermodynamic stability, the kinetic stability (reactivity) is another story and not always free radicals are unstable, think about triplet oxygen molecule, more stable than the singlet one.
If I remember correctly electrons in non bonding orbitals do not stabilize or destabilize the molecule (or ion); so, I do not understand the alleged stability of COT(2-).

By: James Ashenhurst

James Ashenhurst — Mon, 27 Feb 2023 20:36:48 +0000

In reply to Dion Silverman.

The key issue is not so much that they are in non-bonding orbitals, it is more that these non-bonding orbitals are singly-occupied (due to Hund’s rule).

This means that these compounds are very similar in behavior to free radicals – except because there are two free radicals, they are what are known as “diradicals”.

So imagine the instability of free radicals, times two.
(The barrier to recombination of free radicals is basically zero, so these types of compounds very readily combine with each other even if they manage to form – they also react rapidly with oxygen)

By: Dion Silverman

Dion Silverman — Mon, 27 Feb 2023 19:31:42 +0000

Like Dipita Karmakar, I also do not understand why electrons in non-bonding orbitals are destabilising, making the compound anti-aromatic, rather than just non-aromatic. I would think that these electrons contribute to neither stabilisation or destabilisation.

I also do not understand how I would know from the Frost diagram, why, if I take a 4n+2 system, and remove 2 electrons, the result would be the anti-aromatic system expected from a 4n compound. Because all filled orbitals are stabilising? I’m not sure how to see the aromatic stabilisation or lack thereof in this context.

By: James Ashenhurst

James Ashenhurst — Mon, 17 Oct 2022 14:48:00 +0000

In reply to AH.

The Frost circle is mostly useful for visualizing what the energy levels will look like.

Once you know that the energy levels look like, then you fill them up with the number of available pi electrons.

When you do this with cyclobutadiene you end up with one doubly-occupied orbital and two singly-occupied orbitals (unstable). But when you do this with benzene you end up with three doubly-filled orbitals (stable!)

By: AH

AH — Mon, 17 Oct 2022 08:43:36 +0000

Just realised my mistake! It’s the number of pi electrons that tell you if it is aromatic or antiaromatic.
4n+2 is aromatic
4n is antiaromatic.

By: AH

AH — Mon, 17 Oct 2022 08:39:08 +0000

So to conclude: fulfilling all of Huckel’s Rules means a molecule is aromatic or antiaromatic. Drawing a Frost Circle can then determine whether a molecule is aromatic or whether it is antiaromatic.

Is this correct?

By: James Ashenhurst

James Ashenhurst — Wed, 12 Feb 2020 17:55:24 +0000

In reply to Mike Turner. Fixed it, thank you!

By: Mike Turner

Mike Turner — Wed, 05 Feb 2020 18:20:57 +0000

TYPO: Note 2 line 1 “The stability arising from antiaromaticity in the cyclobutadiene dication ” should refer to aromaticity.

Overall this is an excellent series of articles.

By: James Ashenhurst

James Ashenhurst — Wed, 18 Sep 2019 15:08:51 +0000

In reply to Frosty the Circle. Very interesting! Also from the paper: "Comparison with all-trans octatetraene indicates that COT2− has a substantial aromatic stabilization energy (25 kcal mol−1) approaching that of benzene (33 kcal mol−1), but this favorable influence is swamped by Coulomb repulsion. "

By: Frosty the Circle

Frosty the Circle — Wed, 18 Sep 2019 13:02:33 +0000

Your diagram of the cyclooctatetraene dianion shows the added electrons in a non-bonding orbital. With 10 electrons it would obey the numerology of Huckel's rule but the diagram argues against it with the added electrons in the non-bonding orbitals. The compound has been made as a disodium salt but the most recent theory is that it exists because it is stabilized by interactions with counterions: J. Chem. Theory Comput. 2013, 9, 4436−4443.

"Like many multiply charged anions, COT2− exists in isolation only as a short-lived resonance state lying above neutral COT." "....Charge-compensating complexation of COT2−with two sodium cations results in a thermodynamically stable Na2COT compound"