Geometrical Structures of Partially Oxidized Rhodium Cluster Cations, Rh₆O_<i>m</i>⁺ (<i>m</i> = 4, 5, 6), Revealed by Infrared Multiple Photon Dissociation Spectroscopy

Abstract

Infrared multiple photon dissociation (IRMPD) spectra of Rh 6 O m + ( m = 4–10) are obtained in the 300–1000 cm –1 spectral range using the free electron laser for infrared experiments (FELIX) via dissociation of Rh 6 O m + or Rh 6 O m + –Ar complexes. The spectra are compared with the calculated spectra of several stable geometries obtained by density functional theory (DFT) structural optimization. The spectrum for Rh 6 O 4 + shows prominent bands at 620 and 690 cm –1 and is assigned to a capped-square pyramidal Rh atom geometry with three bridging O atoms and one O atom in a hollow site. Rh 6 O 5 + displays bands at 460, 630, 690, and 860 cm –1 and has a prismatic Rh geometry with three bridging O atoms and two O atoms in a hollow site. Rh 6 O 6 + shows three intense bands around 600–750 cm –1 and multiple weak bands in the range of 350–550 cm –1 . This species has a prismatic Rh geometry with four bridging O atoms and two O atoms in a hollow site. Considering that Rh 6 O m + ( m ≤ 3) adopts tetragonal bipyramidal Rh 6 structures, the change at m = 4 to capped bipyramidal and at m = 5 to prismatic geometries results in a reduction of the number of triangular hollow sites. Since NO preferentially binds on a triangular hollow site through the N atom, the geometry change lowers the possibility of NO dissociative adsorption.