Spectroscopic techniques sphere

Final justification for using terms such as inner- or outer-sphere awaits direct spectroscopic confirmation. Electron Spin Resonance, Mossbauer, and Fourier Transform Infrared-Cylindrical Internal Reflection Spectroscopic techniques are being used to establish the structure of surface complexes (see, e.g., McBride, Ambe et al., and Zeltner et al., this volume). The potential for using EXAFS (extended x-ray absorption fine structure) to establish the type of surface complex for Pb + adsorbing onto goethite is currently being undertaken in our laboratory. 

Surface spectroscopic techniques must be separated carefully into those which require dehydration for sample presentation and those which do not. Among the former are electron microscopy and microprobe analysis, X-ray photoelectron spectroscopy, and infrared spectroscopy. These methods have been applied fruitfully to show the existence of either inner-sphere surface complexes or surface precipitates on minerals found in soils and sediments (13b,30,31-37), but the applicability of the results to natural systems is not without some ambiguity because of the dessication pretreatment involved. If independent experimental evidence for inner-sphere complexation or surface precipitation exists, these methods provide a powerful means of corroboration. 

The detailed physicochemistry of the copper sites in the resting enzymes and during the interaction of these sites with various ligands or substrates has been determined by many spectroscopic techniques. Although the copper ions and their first coordination sphere occupy a relatively small fraction of the total molecular volume, in most of these proteins the copper site is either directly involved in binding molecular oxygen or transferring electrons from substrate to acceptor. Therefore, the physicochemistry of the copper sites is reviewed first since this reveals chemistry that ultimately must be related to the mechanism of action. 

Conserved residues, particularly cysteine, histidine, and as-partate/glutamate, can signal likely metal ligands, which is an assignment that can be tested by mutagenesis. Detailed information on the coordination sphere can also be provided by spectroscopic techniques such as X-ray absorption spectroscopy, as weU as UV/visible spectroscopy, or electronic paramagnetic resonance spectroscopy for some metals. 

Figure 2 gives a scheme illustrating some applications in geochemistry and technology where surface reactivity (kinetics of dissolution, catalytic activity, photochemical activity) depends on surface structure, expecially on surface coordination. It has been shown by various spectroscopic techniques [electron-spin resonance (ESR), electron double-resonance spectroscopy (ENDOR), electron-spin echo modulation (e.g., see Motschi, 1987), Fourier transform infrared spectroscopy (Zeltner et al., 1986), and in situ X-ray absorption studies of surface complexes (EXAFS) (Hayes et al., 1987 Brown, 1989)] that inner-sphere... 

The identification and investigation of both processes by spectroscopic techniques is not straightforward. In the latter case, the investigation requires the detection of weak inner- and outer-sphere substrate-ligand interactions, which are difficult to interrogate by most spectroscopic techniques as the perturbation to the metal centre can be quite small. These interactions can, however, be investigated by EPR techniques. By probing these key stmcture-reactivity relationships, one can build an accurate model for enantiomer discrimination and ultimately provide a fundamental basis for improvement in the operation of enantioselective catalysts. 

The use of a spectroscopic technique (specifically, UV-Vis absorption spectrophotometry) to quantify the solvent effect provides doubtless advantages. Thus, the solute is in its electronic ground state, in thermodynamic equilibrium with its environment, so the transition is vertical and the solvation sphere remains unchanged throughout. These advantages make designing a good environmental probe for any of the previous three effects quite easy. 

Several spectroscopic techniques are used for investigating the surface speciation/structure of the deposited precursor species [39-46]. The X-ray absorption fine structure (EXAFS) related techniques are very useful. These allows identification of the kind of atoms surrounding a transition element deposited on the support surface as well as the number of each kind of these atoms and the distances of the transition element from the surrounding atoms. The first and the second coordination sphere can be rather safely probed. Deposition studies on polycrystalline oxidic supports or on monocrystals exposing a certain crystal face have been reported (e.g. the deposition of the... 

ATR (FTIR), DR (UV-Vis-near-IR), NMR, EPR, and RAMAN spectroscopies have been also used to examine whether inner-sphere complexes are formed upon the deposition of TMIS on the surface of an oxidic support and, in some cases, to find the interface speciation. The structure of a deposited TMIS could be changed upon drying. Thus, the apphcation of a spectroscopic technique before drying is in some cases critical. In this respect ATR is very useful because it allows the strong IR absorption of the water molecules to be overcome and interfacial species to be identified. 

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