Thermal Ring-Opening Dynamics of 1,3-Cyclohexadiene Investigated Using High Harmonic Generation

The ring-opening dynamics of 1,3-cyclohexadiene (CHD) has been investigated through three unique viewpoints using high harmonic generation. The first approach involves time-resolved photoelectron spectroscopy (TR-PES) using a high harmonic light source to study the electronic states of CHD. The second approach is time-resolved high harmonic spectroscopy (TR-HHS) to reveal the transient molecular structures. The third approach is soft X-ray transient absorption spectroscopy (SX-TAS) to examine the local electronic states. By analyzing the dynamics observed through these three methods, we have proposed a ring-opening process in the thermal process. Among these three approaches, SX-TAS has provided us with a unique opportunity to gain insight into the molecular structure during the ring-opening reaction. By observing the shifts of K-edge absorption of the carbon atom from the 1s orbital to vacant molecular orbitals around 285 eV at a delay between 340 and 600 fs, we have identified that the molecular structure follows the disrotatory pathway, which is consistent with the prediction by the Woodward-Hoffmann (WH) rules. This confirms that the molecular-orbital symmetry is dynamically conserved in the ring-opening reaction of CHD molecules.