The combination of such simulations of nonadiabatic dynamics with experimental timeresolved spectroscopy investigations can serve as a powerful tool that can provide deep insight into nonadiabatic reaction mechanisms. Additionally, it provides the opportunity to mutually benchmark the interpretation of experimental observations and the accuracy of the simulation methods . Such combined investigations have, therefore, become a standard approach to studying nonadiabatic dynamics in the gas phase . An important aspect in such studies is the comparison of experiment and simulation on equal footing, which typically requires explicit simulation of the experimental observables. This is now routinely possible for observables such as transient photoelectron spectra. However, care must be taken to understand the limits, approximations, and accuracy of the simulated observables. Otherwise, it cannot be clearly identified if deviations of the simulated and experimental time-dependent signals are due to artifacts created by the simulation of the observable or due to the underlying nonadiabatic wavepacket simulation . Information about the evolution of the nuclear structure of the molecules, which complements spectroscopic studies, can now routinely be obtained by ultrafast X-ray and electron diffraction experiments (94-103). In the following, we demonstrate the power of combined ultrafast diffraction and nonadiabatic wavepacket simulation studies for the electrocyclic ring-opening reaction of CHD. Furthermore, we explore new perspectives on the electrocyclic reaction mechanism, its stereospecificity, and the limits of the Woodward-Hoffmann description in two CHD derivatives: αPh and αTp.