  The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a
textbook example of electrocyclic reactions in organic chemistry and a model
for photobiological reactions in vitamin D synthesis. Here, we present direct
and unambiguous observation of the ring-opening reaction path on the
femtosecond timescale and sub-{\AA}ngstr\"om length scale by megaelectronvolt
ultrafast electron diffraction. We follow the carbon-carbon bond dissociation
and the structural opening of the 1,3-cyclohexadiene ring by direct measurement
of time-dependent changes in the distribution of interatomic distances. We
observe a substantial acceleration of the ring-opening motion after internal
conversion to the ground state due to steepening of the electronic potential
gradient towards the product minima. The ring-opening motion transforms into
rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene
on the sub-picosecond timescale. Our work demonstrates the potential of
megaelectronvolt ultrafast electron diffraction to elucidate photochemical
reaction paths in organic chemistry.
