We first performed a minimum energy path (MEP) calculation along the S 1 reaction coordinate to determine the dominant mechanistic pathway from the FC-point for 1 and 2. Figure shows the MEP along S 1 and the energies for the corresponding S 0-3 states; it contains 25 geometries. This MEP demonstrates that the S 0 and S 1 states are nearly degenerate by the 25 th step (S 1 -S 0 energy gap of 0.34 eV). The S 0 geometries of 1 and 2 have pseudo aromatic structures with a delocalized π-system; the π CO , π CC bond lengths are 1. Figure shows the MEP along the S 1-3 surfaces for 2. The MEP includes just 6 steps and leads to a final structure (2-MEP-6) with an S 1 -S 0 energy gap of 1.46 eV, substantially larger than that of 1-MEP-25. The large S 1 -S 0 energy gap of 2-MEP-6 suggests that the dominant mechanistic pathway for 2 is towards an S 1 -minimum. We optimized an S 1 minimum using the final MEP geometry as an input (shown in Figure ). 2-S1 is 3.22 eV above 2-S0 and nearly degenerate with 2-MEP-6. The π CC of 2-S1 is visibly pyramidalized, and the vinyl-hydrogens adopt a transrelationship. These results suggest that the S 1 -pseudo anti-aromatic electronic structure at the FCpoint is relieved by pyramidalizing all carbons in the ring-especially the vinyl carbons-and results in an S 1 -minimum. According to these non-dynamical MEP calculations, pseudo anti-aromaticity is relieved differently in 1 and 2 and leads to different mechanistic outcomes. Indeed, the S 1 -MEPs of 1 and 2 lead directly to a conical intersection and an S 1 -minimum, respectively. This difference may result from the relative facility by which 2 can achieve a trans-configuration compared to 1. Pyramidalization of 1 would lead to highly strained trans-cyclooctene-like geometries. Transcyclooctene is higher in energy than cis-cyclooctene by 16 kcal mol -1 . To understand the dynamical effects of this reaction and fully enumerate all possible mechanistic pathways, we performed non-adiabatic molecular dynamics (NAMD) simulations starting from the S 1 FC region for 1 and 2.