products, which are the orbitals occupied by the two o and two T electrons of the products. This causes the ground-state configuration of the reactants( I)to evolve into an excited configuration(o T*)of the products. This, in turn, produces an activation barrier for the thermal disrotatory rearrangement (in which the four active electrons occupy these lowest two orbitals)of 1, 3-butadiene to produce cyclobutene If the reactants could be prepared, for example by photolysis, in an excited state having orbital occupancy TItT2T3, then reaction along the path considered would not have any symmetry-imposed barrier because this singly excited configuration correlates to a singly-excited configuration otT*l of the products. The fact that the reactant and product configurations are of equivalent excitation level causes there to be no symmetry constraints on the photochemically induced reaction of 1, 3-butadiene to produce cyclobutene. In contrast, the thermal reaction considered first above has a symmetry imposed barrier because the orbital occupancy is forced to rearrange(by the occupancy of two electrons from I, =T*to '=I) from the ground-state wave function of the reactant to smoothly evolve into that of the product. Of course, if the reactants could be generated in an excited state having a t 3 orbital occupancy, then products could also be produced directly in their ground electronic state. However, it is difficult, if not impossible, to generate such doubly-excited electronic states, so it is rare that one encounters reactions being induced via such states It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. What the above Woodward Hoffmann symmetry treatment addresses is whether there will be symmetry-imposed15 products, which are the orbitals occupied by the two s and two p electrons of the products. This causes the ground-state configuration of the reactants (p1 2 p2 2 ) to evolve into an excited configuration (s 2 p* 2 ) of the products. This, in turn, produces an activation barrier for the thermal disrotatory rearrangement (in which the four active electrons occupy these lowest two orbitals) of 1,3-butadiene to produce cyclobutene. If the reactants could be prepared, for example by photolysis, in an excited state having orbital occupancy p1 2p2 1p3 1 , then reaction along the path considered would not have any symmetry-imposed barrier because this singly excited configuration correlates to a singly-excited configuration s2p1p*1 of the products. The fact that the reactant and product configurations are of equivalent excitation level causes there to be no symmetry constraints on the photochemically induced reaction of 1,3-butadiene to produce cyclobutene. In contrast, the thermal reaction considered first above has a symmetry￾imposed barrier because the orbital occupancy is forced to rearrange (by the occupancy of two electrons from p2 2 = p* 2 to p 2 = p3 2 ) from the ground-state wave function of the reactant to smoothly evolve into that of the product. Of course, if the reactants could be generated in an excited state having p1 2 p3 2 orbital occupancy, then products could also be produced directly in their ground electronic state. However, it is difficult, if not impossible, to generate such doubly-excited electronic states, so it is rare that one encounters reactions being induced via such states. It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. What the above Woodward￾Hoffmann symmetry treatment addresses is whether there will be symmetry-imposed