Vibrational spectroscopy site symmetries

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

It must, however, be borne in mind that minor shifts and weak splittings of the IR bands may arise on account of lowering of site symmetry because of strong lattice effects or of coupling of vibrations between perchlorate groups or from a purely isotopic effect within the group (15, 16). For example, the broad and strong band due to the v mode of ionic perchlorate is often split because of lattice effects. Despite these limitations, with a little care and caution, coordinated and noncoordinated perchlorate(s) are conveniently identified by IR spectroscopy. 

The IR spectrum of Ba3[BN2]2 shows low site symmetry for the BN23 groups.68 Vibrational data for Eu3[BN2]2, however, were interpreted in terms of discrete BN23- units of D jh symmetry.69 IR data were reported for a 1,3,2-oxazaborolidine dimer derived from (V)-a,a-diphenylprolinol.70 The IR and Raman spectra of the new adduct P8012.2BH3 included vPB at 565 cm-1 (IR), 574 cm-1 (Raman), as well as characteristic vBH bands.71 High-pressure Raman spectroscopy was used to follow pressure-induced phase transitions for B12As2.72... 

Vibrational-spectroscopy studies indicate that the CO stretching frequency is sensitive to the symmetry of the CO adsorption site (see Table 6.1). Adsorption on a top site leaves CO with a high frequency of vibration, although about 200 cm (4 X 10 " J) lower than in the gas phase. Chemisorption in a threefold site lowers the CO vibration frequency the most, nearly to that of a C—O single bond in an alcohol or ether, for example. 

With regard to the identification of adsorbed molecular species, vibrational spectroscopy plays a key role. For determining the stoichiometry of a molecule other methods are better suited (e.g. XPS), but the chemical state of an adsorbed molecule can be best identified by vibrational spectroscopy. This is in part due to the fact that a vast amount of data exists for bulk compounds. For example the comparison of C-O stretch frequencies in metal-organic compounds like nickeltetracarbonyl, Ni(CO)4, with corresponding data for the surface species allows important conclusions to be drawn about the nature of the molecular adsorbate. In many cases the number of modes observed in vibrational spectroscopy provides direct information on the symmetry of the adsorption site. It has been found that in many cases the frequency of internal stretching modes shows a correlation with the adsorption site. For example the internal vibration... 

IR absorption and Raman spectroscopy are also well suited to the application of uniaxial stress techniques. Stress-induced splittings of the vibrational bands give information about the symmetry of the defect. In favorable cases, it has been possible to determine the kinetics of the H motion between equivalent sites around an impurity from a study of the alignment of the H-related complexes that can be induced by stress. 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

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