Polyhedral Vanadium Oxide Cages: Infrared Spectra of Cluster Anions and Size‐Induced d Electron Localization

Abstract

The size-dependent properties of transition metal oxide clusters have been intensely studied not only because of interest in this peculiar state of matter, but also because of their relevance as building blocks for nanostructured materials. Vanadium oxides, in particular, are important in supported catalysts, [1] as cathode materials in lithium batteries, [2] in bolometric detectors, [3] and as ferromagnetic nanotubes. [4] While the structural characterization of vanadium oxide clusters deposited on surfaces [5] has reached atomic resolution, it remains a major experimental challenge in the gas phase. [6] Infrared photodissociation [7] paired with quantum chemistry is currently the most generally applicable approach for cluster ions even though it requires intense and tunable infrared radiation sources. Below 2000 cm À1 , in the fingerprint region of metal oxide clusters, only free-electron lasers meet these demands. [8] Herein we report the first experimental infrared spectra of transition metal oxide cluster anions in the gas phase. We combine infrared multiple photon dissociation (IRMPD) spectroscopy with density functional theory (DFT) to characterize the geometric and electronic structures of a representative series of vanadium oxide cluster anions, (V 2 O 5 ) n À (n = 2, 3, or 4). [9] Compelling evidence is produced that these anions have the polyhedral cage structures that have been predicted before, but have eluded spectroscopic detection until now. [10,11] Evidence is also found for a size-induced localization of the extra electron in this series of anions.The IRMPD spectra of mass-selected V 4 O 10 À , V 6 O 15 À , and V 8 O 20À are shown on the left in Figure 1. They were measured by irradiating vibrationally cold, mass-selected parent ions with intense, tunable IR radiation from the free-electron laser FELIX [12] and monitoring the mass-selected fragment ion yield as a function of laser wavelength. Only when the radiation is resonant with a fundamental vibrational transition can the cluster ions absorb photons, thereby initiating a sequential multiphoton absorption process [13] which leads to heating of the cluster ion and eventually to photodissociation. The simplicity of the V 4 O 10 À spectrum is striking and immediately suggests a structure of higher symmetry with degenerate transitions. The dominant feature is a single, rather narrow intense band at 990 cm À1 . Based on our previous measurements on vanadium oxide cluster cations it is assigned to a vanadyl stretching mode. [14] The weaker signal below 750 cm À1 is attributed to a VÀOÀV stretch. The IRMPD spectrum of V 8 O 20À is markedly different. While the vanadyl band stays nearly unchanged, a new band, much broader and roughly four times stronger than the vanadyl band, is observed centered at 870 cm À1 . The appearance of the V 6 O 15 À spectrum is intermediate between the spectra described above. An intense vanadyl band, somewhat broader and red-shifted, is followed by a four times less intense band at 830 cm À1 . No signal is observed bel...

Isomorphous Substitution in Bimetallic Oxide Clusters

“…Their simulated infrared absorption spectra agree satisfactorily with the experimental IRMPD spectra, yielding the following assignment of the three absorption regions. The narrow bands at 975, 1000, and 1005 cm ÿ1 for n 2-4 are formed by the 2n ÿ 1 V --O and Ti ÿ --O stretching modes, in good agreement with the 975 cm ÿ1 stretching modes of compounds with titanyl groups in solution [24] and with stretches of vanadyl groups in anionic V 2 O 5 ÿ n [4] and neutral V 2 O 5 n (see below) clusters. The broad bands with maxima at 820 cm ÿ1 (V 3 TiO ÿ 10 ), 880 cm ÿ1 (V 5 TiO ÿ 15 ), and 885 cm ÿ1 (V 7 TiO ÿ 20 ) are due to the symmetric and antisymmetric stretches of the M-O-M bridges.…”

“…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.…”

“…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.…”

“…Recently, photodissociation spectroscopy [3][4][5][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.…”

“…Mass spectra of [4,[15][16][17]. Investigations on neutral clusters using postionization, however, give a lower oxygen to metal ratio [9,18], most probably because of the higher ionization potential of highly oxidized clusters [17].…”