Communications: The structure of Rh8+ in the gas phase

Abstract

The geometric structure of the Rh8+ cation is investigated using a combination of far-infrared multiple photon dissociation spectroscopy and density functional theory (DFT) calculations. The energetic ordering of the different structural motifs is found to depend sensitively on the choice of pure or hybrid exchange functionals. Comparison of experimental and calculated spectra suggests the cluster to have a close-packed, bicapped octahedral structure, in contrast to recent predictions of a cubic structure for the neutral cluster. Our findings demonstrate the importance of including some exact exchange contributions in the DFT calculations, via hybrid functionals, when applied to rhodium clusters, and cast doubt on the application of pure functionals for late transition metal clusters in general.

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

“…5 PBE1 favored slightly higher multiplicities than PBE, presumably due to Hartree-Fock's favoring of openshell systems. 56 As we have previously reported, 32 the choice of the PBE or PBE1 functionals changed the favored geometry of the Rh + 8 cluster from cubic to a close-packed bicapped octahedron (bc-oh), presumably due to the inclusion of a portion of partial exact exchange in PBE1. We have found similar effects for most cluster sizes in the range (n = 6-12) that we discuss here, where it was seen that geometries with square or cubic motifs were favored by PBE whilst close-packed geometries were favored by PBE1.…”

“…We have found similar effects for most cluster sizes in the range (n = 6-12) that we discuss here, where it was seen that geometries with square or cubic motifs were favored by PBE whilst close-packed geometries were favored by PBE1. Given the much better agreement obtained with the PBE1 calculations 32 we will only show these results here, with the results of the PBE calculations presented in the electronic supplementary information (ESI). 57 The results of our calculations at the TPSSh/def2-TZVP level were generally similar to those at the PBE1 level.…”

“…31 We have recently reported a combined far-infrared multiple photon dissociation (FIR-MPD) spectroscopy and DFT study of Rh + 8 , from which we determined the structure to be a bicapped octahedron (bc-oh). 32 This study was a significant advance, since previous calculations (in the absence of experimental data) had suggested that the cube structure was the global minimum. The use of hybrid exchange-correlation functionals was pivotal to the success of our previous study.…”

“…32 For completeness, here we show the experimental spectrum and the calculated spectra at the PBE1/LANL2DZ level (see Fig. 3).…”

Probing C–O bond activation on gas-phase transition metal clusters: Infrared multiple photon dissociation spectroscopy of Fe, Ru, Re, and W cluster CO complexes

“…However, most of these predictions are based on the assumption of cubic cluster structures, 43,44 which may not be the real ground-state structures but merely an artifact of neglecting exact exchange in the density functional theory calculations. 45,46 Clearly, a better understanding of the bare ruthenium clusters is required before an explanation of the observed trends in ν(CO) can be given. …”