The Nonadiabatic Nature of the Substituent Effects in Azobenzene

ABSTRACT The mechanism of thermal Z → E isomerization in azobenzenes has been debated for nearly a century, with inversion, rotation, and nonadiabatic pathways proposed to account for the nonlinear substituent dependence of the reaction rate. Here, we combine systematic kinetic analysis with temperature‐dependent Eyring and isokinetic evaluations to experimentally evaluate the origin of this behavior. A series of para ‐substituted azobenzenes exhibits uniformly negative entropies of activation, suggesting a single nonadiabatic rotational mechanism is operative across all substituents. We found that the characteristic “V‐shaped” Hammett correlation of azobenzene arises not from a mechanistic change, but from the inadequacy of the σ p scale to describe the stabilization of the open‐shell, diradicaloid species involved in the nonadiabatic pathway. The Creary σ· radical parameter restores linearity, confirming that both electron‐donating and electron‐withdrawing substituents increase the reaction rate, stabilizing the diradicaloid species. Complementary calculations using different multireference spin‐flip and single‐reference approaches reproduce the experimental trends and support the predominance of the nonadiabatic pathway, whereas density functional theory (DFT) systematically fails to reproduce these trends.