Talk:Phenalene

Isomers

There are four possible isomers for this framework, depending on which position is the sp³ carbon bearing an additional H. This article is about the 1H isomer, which has a naphthalene and an alkene in the third ring conjugated to it. The other "simple Lewis structure" is the 9bH isomer, which has pi bonds conjugated around the whole periphery of the three rings and a CH at the center. There are two more that cannot be drawn in a non-delocalized simple form. The 2H isomer is similar to the 1H, but has the tetrahedral CH₂ unit in the third ring at the non-adjacent position to the naphthyl portion. And the 3aH isomer has an H at one of the ring-fusion sites around the periphery. Lead ref/review:

• "The isomers of phenalene and their singlet and triplet states: A Hartree–Fock and density functional computational investigation". Journal of Molecular Structure: THEOCHEM. 863 (1–3): 50–54. 2008. doi:10.1016/j.theochem.2008.05.017.

DMacks (talk) 20:54, 18 June 2017 (UTC)

• 
  1H-phenalene
• 
  9bH-phenalene
• 
  2H-phenalene
• 
  3aH-phenalene

Thanks User:EdChem for moving the article to an isomer-specific name. DMacks (talk) 11:42, 19 June 2017 (UTC)

None of the other isomers are stable. So there's no reason to specify which one. OrganoMetallurgy (talk) 23:23, 16 August 2017 (UTC)

"Stability" is not the only (or maybe not even a major) concern, and I don't see support for this claim either. Instead, the ref I cite above supports that 2H- is even more stable than 1H-. I agree that 1H- is apparently only one that has been synthesized. The article is solely about one isomer, and there are others that are of citeable academic interest. User:EdChem, do you think there is value in having a parent article about them, or is there really only one worth writing about? DMacks (talk) 02:02, 17 August 2017 (UTC)

We also have Phosphaphenalene as a derivative of multiple isomers, so I think there is organizational value in having a class (non-isomer-specific) article about the parent hydrocarbon. DMacks (talk) 02:12, 17 August 2017 (UTC)

DMacks, according the the ref you site says only the 1H-isomer has been investigated, that neither the 2H- or the 3aH-isomers have ever been reported, and that all the reports of the 9aH isomer appear to be mistakes. OrganoMetallurgy (talk) 02:23, 17 August 2017 (UTC)

At the time the article was written, yes, but then that article itself does ab initio of the others. Hence the others have now been investigated theoretically. Otherwise you and I couldn't be debating the relative stabilities as such... DMacks (talk) 08:34, 17 August 2017 (UTC)

Also the the ref you cited above doesn't support the claim that the 2H-isomer is more stable, instead it says the the 2H-isomer in the triplet state is the second most stable isomer while the 1H-isomer in the singlet state is the most stable. OrganoMetallurgy (talk) 02:38, 17 August 2017 (UTC)

My mistake, I was looking at the HF not DFT calculation results there. Or triplet vs triplet for each, rather than "lowest among singlet vs triplet" for each. DMacks (talk) 08:34, 17 August 2017 (UTC)

I should have been a bit clearer earlier and said that none of the other isomers are notable, which is likely related to their probable instabilities. OrganoMetallurgy (talk) 14:07, 17 August 2017 (UTC)

After doing a bit more research, I've found that there have been some reports of 2H-phenalene. However they don't call it 2H-phenalene, they call it 1,3-perinaphthadiyl. OrganoMetallurgy (talk) 23:13, 17 August 2017 (UTC)

Here's what I've gathered from what I read. Matrix isolated triplet 1,3-perinaphthadiyl, formed by photolysis of 6b, 7a-dihydro-7H-cycloprop[a]acenaphthylene (which is also called naphthocyclopropane, or cyclopropanoacenaphthylene, among other names), has been studied by ESR, uv-vis, and infrared spectroscopies at low temperatures. It has a triplet ground state and slowly isomerizes to phenalene at 77 K (more rapidly higher temperatures) . Singlet 1,3-perinaphthadiyl is much less stable than triplet 1,3-perinaphthadiyl and undergoes ring closure to form 6b, 7a-dihydro-7H-cycloprop[a]acenaphthylene. I'll add citations later, when I'm less busy in RL. DMacks, if you think it's notable, you can create an article on it. OrganoMetallurgy (talk) 00:13, 19 August 2017 (UTC)

Good find, especially a literature synonym! In that case, I would now support keeping 1H at phenalene (common name of it, most covered in lit at this point, other isomers do/may have other names to provide natural disambig), and if we wanted an article on the class, it could be at a plural title. DMacks (talk) 04:13, 19 August 2017 (UTC)

Here's some sources.

• Polarized IR spectroscopy of a triplet 1,3-biradical: the structure of a methylene-bridged 1,8-naphthoquinodimethane doi:10.1021/ja00267a066
• The behavior of the singlet and triplet spin states of methylene-bridged 1,8-naphthoquinodimethane with O₂ doi:10.1016/0040-4020(82)80165-8
• Reviews of Reactive Intermediate Chemistry doi:10.1002/9780470120828.ch10

OrganoMetallurgy (talk) 19:13, 20 August 2017 (UTC)