Fig. 1: Schematic description of the observed electrocyclic ring-opening dynamics of α-terpinene.
After photoexcitation to the first excited state (S1), the molecule relaxes along a coordinate representing deplanarization with respect to the reactant double bond positions and planarization with respect to the product double bond positions into the pericyclic minimum. The pericyclic minimum is close to, but separated by a shallow barrier from a conical intersection (S0/S1 CI) with the electronic ground state (S0). Population which relaxes through the CI either returns to the S0 reactant minimum or evolves along a carbon–carbon bond dissociation coordinate RC–C into three S0 minima representing different triene photoproduct isomers labeled with cZc, cZt, tZc, and tZt. Visualizations of representative structures along the reaction coordinate are shown together with specific carbon–carbon distances in yellow and blue. Additionally, the distances are reported by color-coded numbers. Both the structures and the distances are extracted from the simulations. The carbon numbering used in the text is shown in black. The double bond positions are highlighted in the structure visualizations as red bars.
