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We know that in butadiene there are 4 MOs. This is the MO with one node:

HOMO of butadiene

My question is: Why, with one node, can't we have something like this ?

Alternative HOMO of butadiene

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    $\begingroup$ MO symmetries should reflect the symmetry of the molecule. This is not a symmetry element for butadiene. $\endgroup$ – Zhe Mar 17 '17 at 2:20
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A good way to think of this is by looking at forming butadiene from the MOs of ethene, which are an in-phase and an out of phase combination of p orbitals. If we looked at all the combinations of these two MOs of ethene, we obtain the MOs for butadiene

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  • $\begingroup$ yes, thought about that but there must be 8 variants ( 2 for each ethene ,that's 2x2=4 and also their interactions ( node in middle or not ) that's 8 ) .also why pi* of one ethene can't interact with pi of another ( it would be the picture of mine ) $\endgroup$ – Saba Tavdgiridze Mar 16 '17 at 17:57
  • $\begingroup$ The main reason you can't have an MO like that is, going back to the two ethenes example, the orbitals that overlap to form the MO have to be of the same symmetry (ie we couldn't form an MO from one orbital with $\pi*$ symmetry and another with $\pi$ symmetry). Also, we only form as many MOs for butadiene as we put in from the two ethenes, so we can only form 4 MOs total. $\endgroup$ – Tyberius Mar 16 '17 at 18:50

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