Reduced-Dimensional Reaction-Space Analysis of a Trispericyclic Reaction by ReSPer

Post-transition-state bifurcation, which plays a critical role in determining product selectivity, has gained significant attention in synthetic chemistry. A trispericyclic reaction has been reported, producing three different cycloadducts from a single transition state structure. Its mechanisms have been examined through elementary reaction processes and dynamical trajectory simulations. However, because the relevant static reaction paths and dynamical trajectories are embedded in a high-dimensional configurational space, their mutual relationships and the potential-energy features governing product branching are difficult to interpret without choosing a limited set of reaction coordinates. In this study, we use the Reaction Space Projector (ReSPer) method, which mathematically constructs a reduced-dimensional chemical reaction space from a set of reference structures to uncover the dynamical branching mechanisms of the trispericyclic reaction. The two-dimensional reaction space reveals that the reaction path network in the configurational space forms an arrowhead-shaped landscape, differing from the traditionally recognized fork-shaped landscape. Projecting dynamical trajectories onto the two-dimensional reaction space shows that dynamic branching processes occur near the shoulder in the energy profile along the main reaction path from the transition state to the major cycloadduct, where the potential valley orthogonal to the reaction direction flattens out. These results demonstrate that ReSPer provides a useful reduced-dimensional representation for relating static reaction-path networks to dynamical branching trajectories, and may offer new descriptors for future branching-ratio prediction in pericyclic reactions.