A deep UV trigger for ground-state ring-opening dynamics of 1,3-cyclohexadiene

Area of Science:

• Chemical kinetics
• Photochemistry
• Ultrafast spectroscopy

Background:

• 1,3-cyclohexadiene is a key molecule for studying photochemical reactions.
• Understanding excited-state dynamics is crucial for controlling chemical transformations.

Purpose of the Study:

• To investigate the photo-induced kinetics of 1,3-cyclohexadiene at 200 nm excitation.
• To elucidate the relaxation pathways and subsequent reactions following excitation.

Main Methods:

• Ultrafast time-resolved gas-phase X-ray scattering.
• Utilized the Linac Coherent Light Source (LCLS).
• Analysis of scattering anisotropy to determine molecular structure and dynamics.

Main Results:

• Excitation at 200 nm populates 3px and 3py Rydberg states, relaxing to the ground state in 208 ± 11 fs.
• A majority (76 ± 3%) of molecules form vibrationally hot ground-state 1,3-cyclohexadiene.
• Subsequent isomerization to 1,3,5-hexatriene observed with forward and backward rates of 174 ± 13 ps and 355 ± 45 ps, respectively.
• The final hexatriene product exhibits a thermal distribution of rotamers.

Conclusions:

• 200 nm excitation leads to efficient formation of vibrationally excited ground-state 1,3-cyclohexadiene.
• This hot species isomerizes to 1,3,5-hexatriene, demonstrating a distinct reaction channel compared to other excitation wavelengths.
• The study provides insights into the ultrafast dynamics and reaction pathways of 1,3-cyclohexadiene.