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Abstract

The interaction of hydrogen with Al_nRh₂⁺ (n = 10–13) clusters is studied by mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Comparing the IRMPD spectra with predictions obtained using density functional theory calculations allows for the identification of the hydrogen binding geometry. For n = 10 and 11, a single H₂ molecule binds dissociatively, whereas for n = 12 and 13, it adsorbs molecularly. Upon adsorption of a second H₂ to Al₁₂Rh₂⁺, both hydrogen molecules dissociate. Theoretical calculations suggest that the molecular adsorption for n = 12 and 13 is not due to kinetic impediment of the hydrogenation reaction by an activation barrier, but due to a higher binding energy of the molecularly adsorbed hydrogen–cluster complex. Inspection of the highest occupied molecular orbitals shows that the hydrogen molecule initially forms a strongly bound Kubas complex with the Al_11-13Rh₂⁺ clusters, whereas it only binds weakly with Al₁₀Rh₂⁺.