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 (σS), followed by the antisymmetric sum (σA). The two highest energy MOs are formed from linear combinations of the σCH antibonds – highest is the antisymmetric σ*A, preceded by the symmetric σ*A at a slightly lower energy. In the middle of the energetic scale are the two remaining MOs that are the πCC and π*CC of ethene.