One at a time or all at once? Electronic fluxes during a pericyclic reaction in the electronic ground state—exemplified for the degenerate Cope rearrangement of semibullvalene—may proceed either synchronously or asynchronously. Quantum simulations show that the mechanism is determined by the preparation of the reactants, for example, synchronous at cryogenic temperatures (tunneling) but asynchronous when induced by selective laser pulses (with energy over the barrier).
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