Femtosecond dynamics of retro Diels–Alder reactions: the concept of concertedness

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time studies of the retro Diels–Alder reactions of cyclohexene, norbornene and bicyclo[2,2,2]oct-2-ene. The experimental results and theoretical calculations elucidate the influence of conical intersections on ground-state reaction trajectories. We address the issue of concertedness on the actual time scale of the nuclear motion.