Correlation vs. Correspondence

The terms correlation and correspondence that were so prominent in the preceding discussion, are not quite synonymous. The distinction is illustrated by a comparison of Fig. 5.2 with Fig. 5.3, the correlation diagram actually calculated for the hexatriene-cyclohexadiene interconversion in the simple HMO approximation. 

The central role assigned by OCAMS to nuclear motion suggests that the alternative modes of desymmetrization capable of allowing polyene cyclization might profitably be described as symmetry coordinates with different irreps in C y. b for disrotation and U2 for conrotation. These are illustrated for the cyclization of butadiene in (a) and (b) respectively of Fig. 5.4. 

The four CH bonds of cyclobutene interconvert under the four sym-ops of C2v and their linear combinations span its four irreps. In butadiene, all four bonds are in the molecular plane and come in two distinct pairs, an inner and an outer as a result, only those irreps appear that are symmetric to reflection in cr(i/z), i.e. Ui and 62 each one twice. 

Few organic chemists would contest the proposition that the CH-bonding electrons remain localized in the CH bonds as the methylene groups rotate.  

The CH-bonding orbitals would therfore be expected to correlate across the diagram. This can be accomplished either by a conrotation, which induces 

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