Cycloadditions: the Diels-Alder reaction (cont'd)

From this example it can easily be seen that both homo/lumo combinations have the right symmetry for overlap. The homo-butadiene/lumo-ethylene MO's are displayed on the right: the right colours match as the orbitals approach eachother.
	The same holds for the lumo-butadiene/homo-ethylene combination, shown left. The red lobe of the ethylene homo overlaps with the red lobes on C1 and C4 in butadiene.
In the first stages of the reaction the newly formed occupied MO's will closely resemble the original ones. Gradually their shape will change, as more of the empty orbitals mixes in. At 2.3 Å we see how the lobes at C1 and C4 extend towards the ethylene carbons. (Click figure for 3D vrml file) At 2.1 Å this effect becomes more pronounced. By extrapolation we can expect that in the end the asymmetric combination of two new sigma orbitals (the new C-C bonds) will emerge. For the symmetric combination (lumo-butadiene/homo-ethylene) such a picture is not so clear, because other symmetric orbitals (e.g. the lowest butadiene pi MO) mix in as well. In the product this combination corresponds to the symmetric combination of sigma orbitals (again combined with other symmetric orbitals).	2.3 Å 2.1 Å