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Question about molecular orbital theory

  1. Apr 11, 2012 #1
    I know that the sigma 2p bonding orbital could be less/higher in energy than the pi 2p bonding (based on if it is C2,N2,B2 etc), but

    Why is the sigma 2p antibonding orbital always higher in energy than the pi 2p antibonding one?
     
  2. jcsd
  3. Apr 12, 2012 #2

    DrDu

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    When you neglect in a first step the s orbitals, the sigma bonding p orbital would be below the pi bonding p orbitals. However, in the second step the sigma bonding p orbital mixes with the sigma bonding s orbitals which shifts them up in energy. Also the antibonding p orbital can be shifted up by that mechanism, although the effect is weaker due to their larger energetic separation and it would not change the ordering of the orbitals.
     
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