[Calafalas]

I'm struggeling with a problem with Molecular Orbitals (MO's) in Diels-Alder reactions. With [4+2] cycloaddition reaction, you can use Frontier Molecular Orbital calculations to predict the reactivity of the seperate components towards the adduct. What I'm interested in, is to have a qualitative prediction of the retro Diels Alder temperature based on FMO calculations. For example, I know that the furan (diene) and maleimide (dienophile) tend to have rDA temperatures of about 110-170^oC and cyclopentadiene with cyclopentadiene has rDA temperatures of about 150-215^oC.
Of course, I can calculate the HOMO and LUMO energies of these components and the adducts. But how can I use these numbers to prove/show that the CPD-CPD reaction has a more stable adduct (and thus a higher rDA temperature) than the adduct of furan/maleimide ?
I will include the energies that I have calculated with HyperChem:

	HOMO	LUMO
Propyl CPD	-9.237	0.313
adduct	-9.530	0.955

	HOMO	LUMO
Propyl furan	-9.229	0.647
maleimide	-8.896	-1.341
adduct	-8.839	0.085

[opsomath]

Off the top of my head, the stability should be related to the HOMO-LUMO gap. That makes sense, as the gap in CPD is about 9.55, and the gap in the furan-MA is 9.54. What are these units?

[Calafalas]

Sorry, they're electronvolts (eV).
But doesn't the rDA reaction have something to do with difference between the HOMO of the adduct and the HOMO's of the separate components? In my vision, that is the energy gap that must be crossed (by heating the adduct) in order to obtain the separate components. Or is this a wrong vision?

[Calafalas]

Furthermore, if a low HOMO-LUMO energy gap ( :delta: E) results in a high reactivity. Does that automatically mean a more stable bond? And does THAT automatically result in a higher rDA temperature ?

[Calafalas]

*Ignore me, I am impatient*
Please.. Help me out. It's important to me, and I really cannot find the answer anywhere else!

[Calafalas]

Please. Is there anyone who knows more about this?   I really need to know which of the rDA temperatures should be higher based on the HOMO-LUMO calculations.