Rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction imaged in real-time

Electrocyclic reactions proceed through critical geometries, which are known as pericyclic transition states in thermal reactions and pericyclic minima in photochemical reactions. Here, the authors image the structure of a pericyclic minimum in real time using a combination of ultrafast electron diffraction and ab initio dynamics simulations. Electrocyclic reactions are characterized by the concerted formation and cleavage of both sigma and pi bonds through a cyclic structure. This structure is known as a pericyclic transition state for thermal reactions and a pericyclic minimum in the excited state for photochemical reactions. However, the structure of the pericyclic geometry has yet to be observed experimentally. We use a combination of ultrafast electron diffraction and excited state wavepacket simulations to image structural dynamics through the pericyclic minimum of a photochemical electrocyclic ring-opening reaction in the molecule alpha-terpinene. The structural motion into the pericyclic minimum is dominated by rehybridization of two carbon atoms, which is required for the transformation from two to three conjugated pi bonds. The sigma bond dissociation largely happens after internal conversion from the pericyclic minimum to the electronic ground state. These findings may be transferrable to electrocyclic reactions in general.

Automated summary

Combining ultrafast electron diffraction and ab initio dynamics simulations, the authors visualize the structure of a pericyclic minimum, also known as a pericyclic minimum, in real time in a photochemical reaction. Electrocylic reactions involve the simultaneous formation and cleavage of sigma and pi bonds through a cyclic structure. This research provides experimental evidence for the importance of this structure in electrocyclic reactions.