Mapping the Product-Dividing Boundary from a Valley-Ridge Inflection to the Product-Connecting Transition State in Substituent-Controlled Bifurcating Reactions

Reaction-path bifurcation is often discussed using valley-ridge transitions (VRTs) along the intrinsic reaction coordinate (IRC), but a VRT reflects the transverse instability of a projected IRC rather than the origin of the product-dividing boundary on the full potential energy surface. Here, we analyze a substituent-controlled bifurcating reaction of α-bromoacetophenone derivatives with hydroxide ion by focusing on the valley-ridge inflection (VRI) as the origin of the product-dividing boundary, using reaction space projector (ReSPer) visualization and natural reaction orbital (NRO) analysis. Projected Hessian analyses along the IRC, with the IRC tangent direction removed, identified transverse modes associated with the VRT near the product-switching region. Small displacements from the VRT structure of the p-CF3 derivative along the VRT mode generated steepest descent paths leading to different products, allowing the product-dividing boundary to be traced from a VRI-like region to the transition state connecting the two product basins. NRO analyses revealed distinct electronic responses on the two sides of the boundary: a two-step electron-flow pattern for the addition path and a direct SN2-type electron flow for the substitution path. These results provide a framework for connecting the PES geometry of reaction-path bifurcation with the associated electronic response.