The lower valence states of linear polyenes: Locations and identifications

The experimental transition energy to the valence A g state of butadiene is used to validate a subset of the theoretical calculations that have been carried out on this molecule. The validated calculated A g transition energies of these calculations are then compared with the experimental B u transition energies to determine the relative state ordering in isolated, unsubstituted butadiene, hexatriene, and octatetraene. The A g origins are concluded to lie below the B u origins for all polyenes except butadiene. Experimental confirmation of this conclusion is presented. The extreme breadth of the A g transition, 1.0 eV, is noted. Some possible implications of this breadth are discussed.