[Compaq]

I try, but I am not able to solve the attached problem.

I am having trouble deciding on the fragment orbitals. I considered dividing in a cross fashion, with the methyl radical, top, and bottom C atoms as one fragment, and then the crossing C atoms as the second fragment. I am also confused about which symmetry operations to consider to make the symmetry-adapted MOs. I chose for my attempt the C2 axis and reflection in the molecular plane.

I know the basic outline of solving this, but I am not able to apply it to the problem:

1) decide on the fragments
2) identify the various orbitals involved, and assign symmetry labels with respect to the chose symmetry operations (that must be common to both fragments)
3) Identify which orbitals that will interact (those of like symmetry)
4) construct MO diagram

I would appreciate some guidance on how to proceed!

Anders

[Corribus]

1) The obvious choice is ethylene and the C₃H₃ radical, approaching like this: ->.
2) Do a simple Huckel treatment of these two fragments using a common point group (use the point group of the combined radical molecule) and assign symmetry labels to the energy states based on orbital coefficients (in practice, you don't need to actually solve the for the coefficients; you should be able to determine them qualitatively - plus or minus, that is - via intuition).
3) Simple once 2) is completed. Orbitals can only interact with other orbitals of same symmetry
4) Also straightforward once 2) is completed.

Then it should be apparent where the radical is located in the "resonance structure" based on the appearance of the HOMO.
If you need more help, shout.