Isomorphous Substitution in Bimetallic Oxide Clusters

Abstract

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted V 2 O 5 nÿ1 VTiO 5 ÿ (n 2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral V 2 O 5 n clusters. Upon systematic exchange of V by Ti atoms in V 4ÿn Ti n O ÿ 10 (n 1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n ÿ 1) for the Ti-rich systems, leading to a quartet ground state for The size-and composition-dependent properties of gas phase transition metal oxide clusters are intensely studied, for example, to aid in the design of functional building blocks for nanostructured materials [1] and to gain a better understanding of the elementary steps in heterogeneous catalysis [2]. A long sought-after goal, in this regard, has been the tailoring of the electronic and geometric structure of clusters by carefully choosing the cluster composition. This approach has been limited up to now by both the availability of adequate cluster sources as well as structuresensitive characterization techniques.Recently, photodissociation spectroscopy [3-6] as well as resonance-enhanced multiple photon ionization [7][8][9] combined with radiation from a tunable infrared free electron laser have proven very useful in probing the structure of monometallic oxide clusters as a function of the cluster size. Here, we present the first systematic compositiondependent infrared study of bimetallic oxide clusters in the gas phase. The structures of titanium-vanadium oxide cluster anions are characterized by infrared multiple photon photodissociation (IRMPD) in combination with density functional theory (DFT) calculations. By sequentially exchanging V with Ti atoms we exploit the control over the clusters composition to investigate its influence on the electronic and geometric structure. Previously, IRMPD studies demonstrated that negatively charged V 2 O 5 ÿ n clusters (n 2-4) form polyhedral cages made of -O 3 V --O units [4]. Opposite to the neutral V 2 O 5 n cages [10], the anionic clusters are open shell systems; they have one unpaired electron in vanadium d states. While the unpaired electron is fully delocalized over all V atoms in n 2, it localizes on a single atom for the larger clusters [4]. Here we extend this study in two series of experiments. First, a single V atom is replaced by a Ti atom, yielding closed-shell V 2 O 5 nÿ1 VTiO 5 ÿ anions isoelectronic with the neutral V 2 O 5 n clusters. Second, the number of atoms is kept constant, but each V atom in V 4 O ÿ 10 is sequentially exchanged by a Ti atom, i.e., a stepwise removal of valence electrons. For ...

Structures and vibrational spectroscopy of partially reduced gas-phase cerium oxide clusters

“…For example, both charged and neutral vanadium oxide clusters form polyhedral cages unknown in bulk V 2 O 5 . 7,8 A similar behavior was shown for bimetallic V-Ti oxide anion clusters 9 and neutral (TiO 2 ) m clusters. 10 For the latter even clusters of sizes m = 9-15 show geometrical structures which do not correspond to any of the known TiO 2 polymorphs.…”

Ferrimagnetic cagelikeFe4O6cluster: Structure determination from infrared dissociation spectroscopy

“…Infrared photodissociation spectroscopy helped to determine the structure of oxide clusters of vanadium, 14 niobium and tantalum 15,16 as well as binary vanadium-titanium oxide clusters. 17 However, the possible existence of magnetic order has not been investigated for either of these systems.Here we report an experimental investigation of cationic iron-oxide clusters in the gas phase. Their vibrational spectra are measured via resonant dissociation of the target clusters ͑Fe n O m + ͒ tagged with molecular oxygen.…”

“…Infrared photodissociation spectroscopy helped to determine the structure of oxide clusters of vanadium, 14 niobium and tantalum 15,16 as well as binary vanadium-titanium oxide clusters. 17 However, the possible existence of magnetic order has not been investigated for either of these systems.…”

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