We show the existence of a dynamic spin equilibrium introduced by intersystem crossing (ISC) between several spin multiplicity states in the Pt 13 cluster. Employing weak-coupling transition probabilities, nonadiabatic transition state theory, and density functional theory, we obtain rate constants for each spin transition and, from these results, we find the equilibrium populations as a function of temperature for each state. At very low temperatures, neither ground-state calculations nor Maxwell−Boltzmann distributions are able to reproduce the experimental magnetic moment of Pt 13 . The origin of such differences, as transition probabilities show, is attributed to tunneling and zero-point energy-assisted ISC, producing mixed spin vibronic states, and to the large orbital contribution to the magnetic moment confirmed by simulated X-ray magnetic circular dichroism. At low temperatures, these physical processes provide the transition mechanism, whereas as temperature increases, quantum effects are less important, and thermal hopping becomes the predominant path. These results reproduce the experimental magnetic moment found in Pt 13 and may explain the origin of anomalous magnetic properties on small metallic clusters.

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