Orbital symmetry selection rules

Recent theoretical and spectroscopic studies indicate that in aliphatic dienes and trienes, excitation to the spectroscopic l1 state usually results in facile twisting about the termini in the stereochemical sense dictated by orbital symmetry selection rules for the appropriate electrocyclic ring closure, motions which are often accompanied by some degree of planarization of the carbon framework. In general, relatively minor distortions... 

Using i/ orbitai and orbital in this form, the integrals implicit in equation (45) are likely to be of the order of the values of C, or C and the band oscillator strengths of the order of their squares. The orbital symmetry selection rule discussed at the end of Section 6.5.1 will have some effect in determining band intensities, but even orbitally forbidden transitions can be expected to appear with finite intensity. 

The generalized orbital symmetry selection rules are given below. The bases of these rules are discussed for each of the major classes of sigmatropic rearrangements. 

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