Ultrafast Diffraction Imaging of the Electrocyclic Ring-Opening Reaction of 1,3-Cyclohexadiene

Changes in electron diffraction patterns are observed on ultrashort time scales upon irradiation of 1,3-cyclohexadiene with femtosecond laser pulses. 1,3- Cyclohexadiene is known to experience a ring opening reaction to hexatriene upon excitation to the 1B2 electronic state. Internal conversion brings the molecule to a saddle point, from where one pathway leads back to cyclohexadiene, while another path generates 1,3,5-hexatriene in one of its isomeric forms. Structural observations are made at picosecond time delays using an ultrashort electron pulse that is diffracted off the nascent product molecules. The diffraction images illustrate that structural observations of prototypical organic reactions can be made in real time, opening a new methodology to study chemical reaction dynamics.