Metal clusters vibrational spectroscopy

Recent advances in the vibrational spectroscopy of metal cluster complexes. I. A. Oxton, Rev. Inorg. Chem., 1982,4, 1-26 (107). 

It is very likely that the metal-insulator transition, the unusual catalytic properties, the unusual degree of chemical reactivity, and perhaps even some of the ultramagnetic properties of metal clusters are all linked intimately with the dynamic, vibronic processes inherent in these systems. Consequently, the combination of pump-probe spectroscopy on the femtosecond time scale with theoretical calculations of wavepacket propagation on just this scale offers a tantalizing way to address this class of problems [5]. Here we describe the application of these methods to several kinds of metal clusters with applications to some specific, typical systems first, to the simplest examples of unperturbed dimers then, to trimers, in which internal vibrational redistribution (IVR) starts to play a central role and finally, to larger clusters, where dissociative processes become dominant. 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

Vlll. Vibrational Spectroscopy of Metal Ouster Adducts lx. Model of Cluster Reactivity... 

VIII. VIBRATIONAL SPECTROSCOPY OF METAL CLUSTER ADDUCTS... 

Vibrational spectroscopy is an important probe used to determine the bonding and structural properties of molecules. Powerful techniques such as electron energy loss spectroscopy (EELS) have been developed, which allow one to obtain the vibrational properties of molecules chemisorbed upon surfaces. Due to low concentration, the highly reactive nature of the clusters, and the large number of possible species which are typically present in the cluster beams used to date, unconventional techniques are required in order to obtain spectroscopic information. One unconventional but powerful technique, infrared multiple photon dissociation (IRMPD), has recently been applied to the study of the vibrational properties of gas-phase metal clusters upon which one or more molecules have been chemisorbed. This same technique, IRMPD, has previously been used to obtain the vibrational spectra of ions, species for which it is difficult to apply conventional absorption techniques. 

Four of the most powerful methods presently applied to elucidate metal cluster geometric structure will be presented in the following. These are mass-selected negative ion photoelectron spectroscopy, infrared vibrational spectroscopy made possible by very recent advances in free electron laser (FEL) technology, gas-phase ion chromatography (ion mobility measurements), and rf-ion trap electron diffraction of stored mass-selected cluster ions. All methods include mass-selection techniques as discussed in the previous section and efficient ion detection schemes which are customary in current gas-phase ion chemistry and physics [71]. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Prior to characterization encapsulation must be ensured and clusters formed outside the cavities must be ruled out. Only then can characterization be reliably carried out. A battery of techniques is available for this purpose, such as C,Xe and metal NMR, EXAFS/XANES, XPS, IR and UV-VIS spectroscopy, electron microscopy, ESR, XRD, etc. Among these methods electronic spectroscopy plays an important role. The UV-VIS spectra reflect changes in the oxidation state of the metal as well as structural changes forced by incarceration and so serve as a valuable tool for the ascertainment of intrazeolite complexation. Although vibrational spectroscopy is most frequently applied, sometimes using the IR spectra as fingerprints for identification, it is inadequate to predict the exact structure of the clusters as these spectra maybe different from those in solution or in the sofid state due to interaction with the zeolite matrix. In any case, reliable characterization requires the combined application of complementary analytical methods. 

Both chemical and physical processes take place during calcination and activation. Ligand decomposition from the metal complex can be monitored with in situ vibrational spectroscopy, for example, using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). In the case of a transition metal ion, the metal oxidation state can be tracked as a function of time and temperature using in situ UV-vis spectroscopy. Finally, the formation of metal clusters and nanoparticles can be monitored using XRD, similar to that described for the synthesis of silicalite-1. 

The other vibrational spectroscopies, although less easily applied, may provide complementary structural information. Raman spectroscopy has been used to detect metal-metal bonds in metal oxide supported osmium [86] and iridium [87] clusters. This method might be expected to find application in the study of zeolite supported metal carbonyl dusters, but it is still far from routine since samples are subject to destruction by laser beams, and fluorescence often prevents measurement of useful spectra. 

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