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

TL;DR

This study visualizes the real-time structural changes during a photochemical electrocyclic reaction, revealing the rehybridization process leading into the pericyclic minimum and the subsequent bond dissociation.

Contribution

It provides the first experimental observation of the pericyclic minimum structure in a photochemical electrocyclic reaction using ultrafast electron diffraction.

Findings

Rehybridization of carbon atoms dominates the motion into the pericyclic minimum.

Sigma bond dissociation occurs mainly after internal conversion to the ground state.

Structural dynamics observed may be applicable to similar electrocyclic reactions.

Abstract

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.