[ROSETTA]

Hi there, Im wondering if someone can help.
In an electronic interaction between a HOMO and a LUMO, is it likely that the HOMO is always the electron rich molecule and the LUMO is from the electron poor molecule.

2) Does a  smaller energy gap between HOMO and LUMO predict a faster interaction between two molecules.


3) Does the energy of the LUMO indicate anything abou the electron accepting abilities of the molecule. ie the more negative the energy of a LUMO is, the better that molecule is at accepting electrons from a HOMO. 

for example, if you had two molecules, A and B, both electron acceptors , A has a lumo energy of -3eV and B has a lumo energy of -6eV, and the same donor is used in both cases (Homo = +10eV) is it more likely that the interaction between A and the HOMO will occur compared with B, or is this simply referring to the speed of interaction.
Someone please help
Thanks
Rosie

[Valdorod]

Rossetta,

1.  Usually yes.  The HOMO holds the highest energy electrons and should be subject to attack by electrophiles, while the LUMO provides the lowest energy space for extra electrons and is subject to attack by nucleophiles.

2.  Also usually yes, the smaller the gap the lower the energy required to excite the electron from the HOMO to the LUMO, lower wavelength.  However, the degree to which molecular orbitals mix is proportional to S²/Δε, in this case S is the overlap and Δε is the difference in molecular orbital energies on two species.  Assuming that S is not very different for the orbitals considered then Δε dominates and the smaller energy gap would take precedence.

3. Not always, as in 2, if Δε dominates yes, however, symmetry is very important in HOMO/LUMO interactions and for systems in whish S(from above dominates) then the smaller energy gap will not always lead to an interaction.  Reactions are allowed or forbidden depending on orbital symmetry.  You can look up the "Woodward-Hoffman" rules to determine when reactions are allowed.

Valdo

[ROSETTA]

thank you so much that explanation is great, I am off to look up the woodward hoffman rules
Thanks
Rosie

[ROSETTA]

Hi, looked up woodward hoffman rules, however, I am still unclear.  If we had three isomers.  Ortho, meta and para which represent the LUMO, and we have a pi donor, how can i tell which reaction is most likely to proceed.  The lowest energy gap is for the lumo of the ortho isomer and the homo of the donor.  From my understanding that is the reaction that would proceed.  However, I am not clear on the W-H rules.
Hope somone can clarify
Many thanks
Rosie

[Valdorod]

Rosetta,

While, I am not well versed on the rules myself, they explain unexpected cases for some reactions.  For the example that you are asking, they would not apply, since whether it is ortho, para, or meta, it is the same group, thus the orbitals would be the same.  For your case you would just proceed normally with the most likely reaction being the one with the smallest gap.

However, par of the rules try to explain cases in which the lowest gap does not lead to a reaction due to symmetry constrains.

For example the HOMO in cis-1,3 butadiene is able to interact with the LUMO in ethene to form cyclohexene (A Diels-Alder cycloaddition).

On the other hand the HOMO in one ethene and the LUMO in another ethene do not combine to form cyclobutane, even though the energies are close together to indicate otherwise, in this case the symmetry is not the correct one.

Valdo