Figure . Visualization of the conservation of orbital symmetry in thermally and photochemically allowed electrocyclic ring-opening reactions of 1,3-cyclohexadiene (CHD). The two pathways lead to different reaction products in substituted CHD (see bottom scheme). In the conrotatory ring-opening mechanism (indicated by blue text and left arrow), a C2 rotation axis (visualized together with the highest occupied molecular orbital) is preserved. In a disrotatory reaction mechanism (indicated by purple text and right arrow), a mirror plane (visualized together with the second highest occupied molecular orbital) is preserved. The conservation of orbital symmetry permits only the correlation of orbitals in the reactant and product, which transform identically with respect to the preserved symmetry operation. In the disrotatory pathway, all occupied reactant and product orbitals correlate with each other. Therefore, the pathway is thermally allowed. In the conrotatory pathway, the highest occupied and lowest unoccupied orbitals correlate with each other, which makes the pathway thermally forbidden. If one electron is promoted from the highest occupied to the lowest unoccupied molecular orbital, singly occupied molecular orbitals correlate with each other. Therefore, the pathway is photochemically allowed. The molecular orbital isocontours (far left, center, and far right column) are computed using CASSCF(6,6)/6-31G*. In the case of the disrotatory ring-opening of CHD, the preserved symmetry element is a mirror plane (see a visualization together with the schematic of the second highest occupied molecular orbital in Fig. ). The molecular orbitals can be categorized as either symmetric (red)