A machine-driven hunt for global reaction coordinates of azobenzene photoisomerization*

Azobenzene is an important system which is often studied to better understanding light-activated mechanical transformations via photoisomerization. The C-N=N-C dihedral angle is widely recognized as the primary reaction coordinate for isomerization. We report on a global reaction coordinate to thoroughly describe the reaction mechanism. Our global reaction coordinate includes all of the internal coordinates of azobenzene contributing to the photoisomerization reaction coordinate. We quantify the contribution of each internal coordinate of azobenzene to the overall reaction mechanism. Finally, we provide a detailed mapping on how each significantly contributing internal coordinate changes throughout the energy profile. In our results, the central C-N=N-C dihedral remains the primary internal coordinate responsible for the reaction coordinate; however, we also conclude that the disputed inversion-assisted rotation is half as important to the overall reaction mechanism and the inversion-assisted rotation is driven by four adjacent dihedral angles C-C-N=N with very little change to the adjacent C-C-N angles.