I am working on this reaction: R-H + X· → R· + X-H
I'm curious about the relationship of the SOMO energy of the abstracting radical and the reaction barrier. The radical SOMO is somewhere between the R-H LUMO and HOMO, when X· is electrophilic, SOMO energy is more close to HOMO. because the interaction between the radical SOMO and the R-H HOMO helps to lower the overall barrier, so the lower the radical SOMO evergy, the better. But, is there a theory, maybe, supporting this viewpoint? Is there a numerical relationship between the SOMO energy and the barrier?
Note:
Here is the answer they gave me on computational chemistry list. He said the barrier and orbital gap are related in, eg. Diels Alder reaction. So, is it true in hydrogen abstraction reaction? I am really confused. Please help me.
(by Marcel Swart from CCL)
When you look into the literature of Frontier MO for e.g. Diels-Alder reaction,you can see a good correlation (I seem to remember between the
HOMO(diene)-LUMO(dienophile) and the reaction barrier). And this
was also related to the change in barrier and MO-levels when electron-
withdrawing or -donating groups are added.
Hey,
Even if HAT seems to be a "easy reaction", it is rather tough to accurately describe it.
A crucial question that you should wonder, imho, is about the kind of HAT. Indeed, HAT is mainly related to three mechanisms, depending on your R-H. Indeed, you can have e.g., PCET, ET-PT and SPLET mechanisms (e.g., in polyphenol chemistry). Depending on how you are going to describe your mechanism (e.g., thermodynamics, kinetics), relationships may differ since the driving force might be either an "atom-" or an electron-transfer process.
Amongst others, Prof. Hammes-Schiffer has provided many papers about HAT reactions and mechanisms.
Once you know which mechanism, the relationship could be more easily established (especially in ET-mechanisms)
Hope it helps
Hey,
Even if HAT seems to be a "easy reaction", it is rather tough to accurately describe it.
A crucial question that you should wonder, imho, is about the kind of HAT. Indeed, HAT is mainly related to three mechanisms, depending on your R-H. Indeed, you can have e.g., PCET, ET-PT and SPLET mechanisms (e.g., in polyphenol chemistry). Depending on how you are going to describe your mechanism (e.g., thermodynamics, kinetics), relationships may differ since the driving force might be either an "atom-" or an electron-transfer process.
Amongst others, Prof. Hammes-Schiffer has provided many papers about HAT reactions and mechanisms.
Once you know which mechanism, the relationship could be more easily established (especially in ET-mechanisms)
Hope it helps
You may find this review helpful
Chem Rev. Dec 8, 2010; 110(12): 6939–6960.
Published online Nov 4, 2010. doi: 10.1021/cr1001436
The theory is caled Fronteir Molecular Orbital Theory (FMOT), if you are considering the actual orbitals involved, including symmetry and perhaps charge etc.
If you are thinking more abstractly and considering interactions between non-descript AOs or MOs without consideration of symmetry (or assumed symmetry) you are doing Perturbation Molecular Orbital Theory (PMOT). This came before Molecular Orbital Theory and can be considered a semi-classical method. The most important part is the energy-gap between interacting states and the inverse relationship to interaction energy with the gap.
!!! Often these are mixed without realizing it. If you consider the energy gap betweeen interacting described MOs, then you are doing FMOT-PMOT. !!!
Its important to know the models you are using and if you mix them to do so with full knowledge. This is also a problem when we mix the language of valence-bond theory with MO theory, which is done all the time.
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