Imaging rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction in real-time

We investigate structural dynamics of wavepacket relaxation along pericyclic transition states with a combination of ultrafast electron diffraction (UED) and ab-initio multiple spawning (AIMS) excited state wavepacket simulation. We focused this study on the molecular system alpha-terpinene (αTP, C₁₀H₁₆), which is a derivative of 1,3-Cyclohexidiene (CHD, C₆H₈) by the addition of two substituents, a methyl group and an isopropyl group (replacing the relevant hydrogen atoms). We took the advantage that αTP does not qualitatively alter the photochemical dynamics in comparison of CHD, but the carbons in the substituent groups act as “reporter” atoms in adding signatures of carbon-carbon bond distance to the time-dependent pair distribution functions (PDFs), which were missing in our previous study of CHD. We observed real-time signatures of the structural evolution towards the pericyclic minimum in both measurements and simulations. Detailed analyses from the simulated wavepacket revealed the signatures to be due to the hybridization change from sp³ to sp² configurations largely happening in the excited state prior to bond dissociation to the ring-opening which takes place directly after the internal conversion to the ground state. Our combined experimental and theoretical approaches reflected structural dynamics leading to overlap of the conjugated π-system with the σ-bond prior to its dissociation. In addition, this work also questions the simple picture of the conrotatory motion predicted by the famous Woodward-Hoffmann rules.