[Enthalpy]

Hello you all!

I'm going nuts about the heat of formation of spiroheptanes and spirohexanes. From the data I have, the spirohexane strain energy would be less than the sum of a cyclopropane and a cyclobutane, which I didn't expect.

AM1 confirmed that:

• Geminal methyls at cyclobutane and cyclopropane put themselves wider apart than on neopentane;
• Closing a second ring takes more energy than the first one.

This might be compatible with a smaller strain energy if the C-C bonds are much stronger between a ring carbon and an external one. Your opinion please?

An other option is that I got conned by the heat of vaporization. I take only liquids everywhere, but for cyclobutane and cyclopropane at 298K the meaning isn't trivial.

Presently, I stick to experimental data for spirohexanes, hoping they are accurate even if I don't grasp the values. But I lack data for spiroheptanes, and can even less imagine values if I didn't understand what happens. I don't even have gem-dimethylcyclobutane.

Would you have more data about spiroheptanes please? Without or with methylidenes and other groups, I can make the corrections if needed. But in a known state (liq, gas, T). For instance NIST has some, inaccessible to me.

Thank you!

[Corribus]

An interesting problem, Enthalpy. A cursory search didn't turn up much experimental data on these compounds, though.

[Enthalpy]

I found nearly no experimental data, but don't access paying publications. My single and only source is Rappoport and Liebman which reports a single paper, so there are real chances that the measures are wrong. Meanwhile I've seen several significant inconsistencies among the values reported by Nist.

Having made data tables meanwhile, I don't believe any more that the heat of vaporization plays a role.

One possibility is that the heat of formation lost in branching, about 10kJ/mol for each methyl put at a cyclopropane or cyclobutane rather than in an unstrained backbone, compensates the ring interaction at the spiro carbon of spirohexane. That's not much.

All these heats of formation are imprecise. Combustions release several MJ/mol but a 10kJ/mol error is a lot for my purposes. I wish the heats of formation would be measured by hydrogenolysis for instance.

[Corribus]

Did you find any papers behind a pay wall that may be promising and which I may investigate for you?

[Enthalpy]

That would be nice! In case you access the Nist database for professionals, here are compounds. Not sure they give a heat of formation, combustion or anything usable.
https://webbook.nist.gov/cgi/cbook.cgi?ID=C63783868&Units=SI
https://webbook.nist.gov/cgi/cbook.cgi?ID=C185024&Units=SI
https://webbook.nist.gov/cgi/cbook.cgi?ID=T999922928&Units=SI
https://webbook.nist.gov/cgi/cbook.cgi?ID=T999931214&Units=SI
from that, I could make comparisons and deductions.
If not, I can try to look for other sources.

[Corribus]

https://webbook.nist.gov/cgi/cbook.cgi?ID=C63783868&Units=SI

I can't get past the pay wall for NIST apparently but I do have other resources. E.g., for this compound I found an experimental formation enthalpy in gas phase of 244.4(5.0) kJ/mol, and a strain energy of 364.5(5.0) kJ/mol. The reference is Kolesov et al 1998, J. Chem. Thermodynamics. 30 1455.

Vlaar et al. JOC 2002 67 2485 has some theoretical calculations of the formation enthalpy.

Those articles have some other compounds in them that may be of interest to you as well. 

If I get some time, I'll look into the other compounds you mentioned, although I didn't find much for the second one.