We performed an extensive artificial
intelligence-accelerated
quasi-classical
molecular dynamics investigation of the time-resolved mechanism of
the Diels–Alder reaction of fullerene C60 with 2,3-dimethyl-1,3-butadiene.
In a substantial fraction (10%) of reactive trajectories, the larger
C60 noncovalently 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 the barrier is passed, the bonds take longer
to form compared to the simplest prototypical Diels–Alder reaction
of ethene with 1,3-butadiene despite high similarities in transition
states and barrier widths evaluated with intrinsic reaction coordinate
(IRC) calculations. C60 is mainly responsible for these
differences as its reaction with 1,3-butadiene is similar to the reaction
with 2,3-dimethyl-1,3-butadiene: the only substantial difference being
that the extra methyl groups double the probability of the prolonged
alignment phase in dynamics. These additional calculations of C60 with 1,3-butadiene could be performed via active learning
more easily by reusing the data generated for the other two reactions,
showing the potential for larger-scale exploration of the effects
of different substrates in the same types of reactions.