The Nonadiabatic Nature of the Substituent Effects in Azobenzene

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.