Ultrafast Events in Electrocyclic Ring-Opening Reactions.

Electrocyclic reactions are characterized by the concerted formation and cleavage of multiple σ and π bonds in a molecular system and have been extensively studied since they were introduced by Robert Burns Woodward and Roald Hoffmann in 1965. Recent advances and the integration of time-resolved experiments and nonadiabatic quantum molecular dynamics simulations have transformed the traditional understanding of electrocyclic reactions beyond the Woodward-Hoffmann rules. In this review, we focus on recent studies of 1,3-cyclohexadiene and two of its derivatives, α-phellandrene and α-terpinene, to shed light on the underlying mechanisms of electrocyclic photochemical reactions. We highlight recent progress in ultrafast electron diffraction techniques and the simulation approach of ab initio multiple spawning. Together, these approaches can elucidate molecular structure dynamics from femtosecond to picosecond timescales as well as nuclear and electronic responses at conical intersections.