When ground state energies are different but transition state energies are similar, selectivity will be degraded in the transition state, and poor overall selectivity may be observed. For instance, high selectivity for one ground state conformer is observed in the following radical methylation reaction.
The conformer in which A(1,3) strain is minimized is at an energy minimum, giving 99:1 selectivity in the ground state. However, transition state energies depend both on the presence of A(1,3) strain and on steric hindrance associated with the incoming methyl radical. In this case, these two factors are in opposition, and the difference in transition state energies is small compared to the difference in ground state energies. As a result, poor overall selectivity is observed in the reaction.