Bridging local structure

While the cross-bridge local adsorption site of acetylene on Cu(lll) and Ni(l 11) is essentially identical, on the structurally similar Pd(lll) surface the molecule adsorbs in a hollow site, as illustrated in Figure 1.7. This different adsorption site, first proposed on the basis of quantitative evaluation of NEXAFS spectra [86] and subsequently confirmed by PhD [87], provides a rationale for the significantly different behaviour seen in vibrational spectroscopy [78] for this system. 

The conversion of methanol to hydrocarbons (MTHC) on acidic zeolites is of industrial interest for the production of gasoline or light olefins (see also Section X). Upon adsorption and conversion of methanol on calcined zeolites in the H-form, various adsorbate complexes are formed on the catalyst surface. Identification of these surface complexes significantly improves the understanding of the reaction mechanism. As demonstrated in Table 3, methanol, dimethyl ether (DME), and methoxy groups influence in a characteristic manner the quadrupole parameters of the framework Al atoms in the local structure of bridging OH groups. NMR spectroscopy of these framework atoms under reaction conditions, therefore, helps to identify the nature of surface complexes formed. 

The combined results of EXAFS and Mossbauer effect spectroscopies have been used to deduce the oxidation state and local structure of iron in iron containing Na, K- and H-chabazites. In the mineral Na,K(Fe) chabazite iron is found to be present as oxo- or hydroxo-bridged Fe(III) oligimers, while in the H-chabazite it is found to be present as Fe(III) ions bridged by oxo- or hydroxo-groups to the zeolite frameworks... 

Fig. 4. Local structural rearrangements on cleavage of the influenza HAq precursor protein. HAq (Chen et al, 1998) was superposed on the HA1-HA2 structure (Wilson et al, 1981). The HAq loop that contains tlie cleavage site is marked by small black circles. After cleavage, the HAj amino terminus (HA2-N open circle) and the HAj carboxy terminus HAi (HAi-C open circle) are displaced. The HA2 amino-terminal region is indicated by a thick black line, and the carboxy-terminal region of HAj by a fine dashed line. The thick dashed line represents a salt bridge between the amino terminus of HA2 and an aspartate side chain, shovm in ball-and-stick representation, from the long helix of HA2. The distance between the HAi carboxy terminus and HA2 amino terminus is indicated.

Moreover, the value for the ratio amount of ions released / amount of cationic species adsorbed suggests a disubstituted and possibly trisubstituted structure. On the other hand, the widely accepted high reactivity of the terminal oxygens with respect to the bridging ones leads to the assumption that the former principally participate to the inner sphere complexes. Taking into account the above, we propose a tentative local structure for the inner sphere complexes formed in both cases. This is illustrated in Fig. 3. 

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