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 20 À 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 20 À 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...

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