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Why can different shapes of the same cluster react at different rates?

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cluster Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

Different geometric forms of a cluster with the same elemental composition can exhibit different reaction rates because they function similarly to chemical isomers. Just as molecular isomers possess distinct structures, physical properties, and reactivities, different geometric arrangements of a cluster—even those of the same size—can lead to variations in how they interact with a reactant partner.1

In the context of cluster chemistry, these structural differences are significant because they dictate the specific arrangement of atoms and the availability of surface sites for reactions. While molecular isomers typically do not transform into one another readily, cluster isomers may interconvert with relative ease, sometimes requiring only moderate amounts of energy beyond their formation energy. In some instances, if a reaction is sufficiently exothermic, the energy released can even trigger a phase change, such as melting the cluster from a solid to a liquid during the reaction process.12