Hydroxyl bridging

Hill, J.-R., Freeman, C. M., Delley, B., 1999, Bridging Hydroxyl Groups in Faujasite Periodic vs Cluster Density Functional Calculations , J. Phys. Chem. A, 103, 3772. 

Operando DRIFTS measurements suggest that bridged hydroxyl groups are in extensive interaction with hexane molecules during the reaction even at 553 K. However adsorbed alkene or surface alkoxide could not be detected. These findings questions, whether the Haag-Dessau mechanism [4] gives true description of the alkane activation process over zeolite catalysts. 

The mechanism that is consistent with biochemical, enzymological and structural data involves binding of arginine, in which the side-chain of Glu277 plays an important role, attack of the nucleophilic metal-bridged hydroxyl ion, formation of a neutral, tetrahedral intermediate which is stabilized by the dinuclear Mn(II) centre, and finally proton transfer from Hisl41, followed by release of the two products (Figure 16.5). 

Breit-Winger-Fano representation of edge structure, 34 261 Brewer-Engel theory, 38 62-63 Bridged hydroxyl groups, 34 159-174 acidity versus... 

DRIFT-IR Spectroscopy—Study of Surface Hydroxyl Groups DRIFT-IR spectroscopy at 298 K of self-supported wafers in the region 3500 to 3900 cm indicated the presence of four different types of terminal and bridged hydroxyl groups... 

By measuring the shifts of the various hydroxyl bands of the zeolite, a direct measure of the relative acid site strengths can be made without the need for thermal desorption. Table 4.6 lists the measured hydroxyl band shifts for a variety of hydroxyl groups on different zeolites using low temperature CO adsorption. This data indicates that there is indeed a difference in the intrinsic acid strength of the bridging hydroxyl groups in different zeolites as well as in the same zeolite structure with different framework aluminum content. 

Spectrum 1 illustrates how the interaction with tetramethylbenzene (by hydrogen-bonding) shifts the silanol band in Beta to 3563 cm" and the bridging hydroxyls to even lower frequencies where overlapping absorption from hydrogen-bonded silanols make them difficult to detect In spectrum 1 the bands between 3100 and... 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

Fig. 20. Schematic diagram of indirect carboxylate-zinc interactions through bridging hydroxyl groups, as observed in the (a) zinc proteases and (b) carbonic anhydrases.

Shift to higher wave-numbers of bridged hydroxyls (Fe-OH-Si)... 

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