Surprising new dynamics phenomena in Diels–Alder reaction of C60 uncovered with AI

Our recently developed physics-informed active learning allowed us to perform extensive AI-accelerated quasi-classical molecular dynamics investigation of the time-resolved mechanism of two Diels–Alder reactions. This investigation revealed that despite the high similarity between static transition state geometries in reactions with ethene and fullerene C60 as dienophiles, the dynamics around the transitions are remarkably different. In a substantial fraction (10%) of reactive trajectories, the larger C60 non-covalently attracts the 2,3-dimethyl-1,3-butadiene long before the barrier so that the diene undergoes the series of complex motions including roaming, somersaults, twisting, and twisting somersaults around the fullerene until it aligns itself to pass over the barrier. These complicated processes could be easily missed in typically performed quantum chemical simulations with shorter and fewer trajectories. After passing the barrier, the bonds take longer to form than in the case of the ethene reaction: the consequence of the markedly different topology of the PES region between reactants and products. These effects are not captured by static intrinsic reaction coordinate (IRC) calculations that do not reveal any difference in the barrier widths.