Adsorption Forms of Hydrogen Molecules on Gas-Phase Rhodium Cluster Cations Revealed via Messenger-Complex Infrared Multiple Photon Dissociation Spectroscopy

The structure of hydrogen (deuterium) adsorbed on gas-phase Rh cluster cations, Rhn+ (n = 5–8), was investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. For Rh6D2+, the IRMPD spectrum indicates that one D2 molecule is molecularly adsorbed side-on (η2) to a Rh atom in the octahedral Rh core, although the DFT calculations predict the dissociative adsorption form to be thermodynamically more stable. The energy barrier for the dissociation of D2 on Rh6+ from the molecular adsorption state was calculated to be +0.28 eV, which suggests the kinetic trapping of the molecular adsorption isomer under low-temperature experimental conditions. For Rh6D4+, one D2 molecule is adsorbed molecularly, whereas the other two D atoms are adsorbed separately on bridge sites of the Rh core. For both Rh6D2+ and Rh6D4+ the red-shift of the vibrational wavenumber indicates Kubas bonding, in which the electron density in the bonding σ orbital of D2 is transferred to the Rh atom forming a shallow potential well. Born-Oppenheimer Molecular Dynamics (BOMD) simulations for Rh6D2+ reproduce the band broadening, which can be explained by the fluctuations of the D–D bond length and the D2 orientation. For Rh6D8+, the Rh core adopts a bicapped tetrahedral structure. For RhnD2+ (n = 5, 7, and 8), the IRMPD spectra show that D2 is dissociatively adsorbed in the dominant isomer. The reaction pathways obtained by DFT calculations demonstrate that the dissociation of D2 on Rh7+ proceeds with a lower energy barrier than that for Rh6+; this explains the different adsorption forms of D2 on Rh6+ and Rh7+.