Communication: The structures of small cationic gas-phase platinum clusters

The structures of small platinum clusters [12pt]{minimal}{document}Pt₃₋₅⁺{document} Pt 3−5+ are determined using far-infrared multiple photon dissociation spectroscopy of their argon complexes combined with density functional theory calculations. The clusters are found to have compact structures, and [12pt]{minimal}{document}Pt₄⁺{document} Pt 4+ and [12pt]{minimal}{document}Pt₅⁺{document} Pt 5+ already favor three-dimensional geometries, in contrast to a number of earlier predictions. Challenges in applying density functional theory to 3rd row transition metal clusters are addressed. Preliminary calculations suggest that the effects of spin-orbit coupling do not change the favoured lowest-energy isomers.