"For the analysis of these reactions correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molecular symmetry elements."

Woodward and Hoffmann, "The Conservation of Orbital Symmetry", Verlag Chemie, 1970; p. 114.

Following this statement, Woodward and Hoffmann illustrate several alternative ways of analyzing sigmatropics. We'll look here at the first, which is a sort of perturbation MO treatment of a suprafacial 1,3-sigmatropic migration of a hydrogen.

The analysis begins with the isolated orbitals (on the left): the s bond orbital, containing two electrons, and the p orbital, also containing two electrons.

Mixing the s into the p in a bonding way (lower middle), gives one starting MO. Mixing the two in an antibonding way (top middle) gives the other, which still retains some bonding character. Each of these orbitals holds two electrons.

Finally, migrate the hydrogen, by simply sliding its s orbital from the right to the left.

• From the lower energy starting orbital, we produce a fully bonding MO to contain one pair of electrons.

• From the higher energy orbital we obtain a fully antibonding MO, meaning that the second pair of electrons would have to go into an orbital of significantly higher energy than the one they occupy in the starting situation.