text: The symmetry-imposed barrier heights of group transfer reactions can also be analyzed using correlation diagrams. A model reaction is the transfer of a pair of hydrogen atoms from ethane to perdeuterioethylene shown to the right.
  The only conserved symmetry element in this reaction is the mirror plane through the center of the molecules as shown to the left.
  The molecular orbitals of the system are constructed as symmetric and antisymmetric combinations of σ and σ C–H bonds in ethane and π and π bonds in the deutero-substituted ethene. Thus the lowest energy MO is the symmetric sum of the two C–H σ-bond (σ), followed by the antisymmetric sum (σ). The two highest energy MOs are formed from linear combinations of the σ antibonds – highest is the antisymmetric σ, preceded by the symmetric σ at a slightly lower energy. In the middle of the energetic scale are the two remaining MOs that are the π and π of ethene.
  The full molecular orbital correlation diagram is constructed in by matching pairs of symmetric and asymmetric MOs of increasing total energy, as explained above. As can be seen in the adjacent diagram, as the bonding orbitals of the reactants exactly correlate with the bonding orbitals of the products, this reaction is not predicted to have a high electronic symmetry-imposed barrier.