Unraveling the Electronic Origin of Selectivity in Ambimodal Transition States with Valence Bond Theory

Ambimodal reactions are characterized by a single transition structure that bifurcates toward multiple products, posing a fundamental challenge for understanding the origin of product selectivity. While selectivity emerges along post-transition-state pathways, a direct and chemically intuitive connection between transition-state electronic structure and product distribution remains elusive. Here, we demonstrate that product selectivity in ambimodal reactions can be quantitatively correlated with a purely electronic-structure descriptor derived from valence bond (VB) theory. By analyzing the relative weights of VB structures associated with reaction pathways leading to competing products at the ambimodal transition state, a VB-based descriptor is defined to quantify product selectivity. Across a diverse set of ambimodal reactions, this descriptor exhibits a robust linear correlation with the logarithm of the product ratio obtained from experiment or trajectory simulations. These findings reveal that, although selectivity manifests dynamically after the transition state, the transition-state electronic structure contains an intrinsic electronic bias that correlates strongly with product selectivity and influences subsequent pathway bifurcation. The VB analysis thus provides a chemically intuitive perspective on bifurcating reaction pathways and complements existing approaches on ambimodal reactions.