Much of what is known about chemical-reaction dynamics comes from looking at the decomposition of isolated hot molecules whose energies can be precisely controlled. Those unimolecular reactions can offer insight that’s applicable to the more familiar bimolecular reactions, but without the complications of having to account for the impact parameter and relative orientation of two reacting molecules. As a result, it is possible to study relatively large systems at a level of detail not available with bimolecular methods.

Chemists have long assumed that unimolecular dissociation follows one of two decomposition modalities. The first is dissociation over a potential energy barrier into a pair of smaller molecules, as shown in figure 1a. The barrier arises because of the rearrangement of electrons in breaking old bonds and forming new ones. The second dissociation modality involves simply stretching a bond until it breaks, as shown in figure 1b. The dissociation typically...

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