Vicinal silanols

Isolated silanol Vicinal silanols Geminal silanols... 

In principle, TiCU might react with single silanols, vicinal pairs, or geminal pairs of silanols on silica as shown in reaction 2, 3, and 4. Although these reactions are not shown, it is possible that only one or both of the OH groups of a vicinal pair or geminal pair of silanols might react as depicted for an isolated silanol. 

In principle, TiCU might react with single silanols, vicinal pairs, or geminal pairs of silanols on silica as... 

Silanol sites (SiOH) impart much of the surface behavior of silica and serve as the point of chemical attachment via derivitization processes [4]. Basic agreement has been achieved with respect to the existence of three primary configurations of silanol sites. The orientations are depicted in Figure 32.1 and include isolated or unassociated silanols, vicinal sites that are engaged in... 

Fig. 1. Silanol groups of amorphous silica surface, where 0= Si Q — O and = H (a) isolated, (b) vicinal, and (c) geminal.

A review of the properties of silica as applied to reversed phase separations summarizes a number of issues that have been debated for many years.71 The review categorizes unmodified silanols as free, geminal, vicinal, and internal. The pK, values of silanols average about 7.1, but some silanols may have pK, values as low as 3. As reported in Chapter 4, heavy metal... 

The same surface species is obtained at ambient temperature by the reaction of Bu3SnH and the silanol groups, suggesting that the Sn-H bond is more reactive in this case than the Sn-C bond. The surface reaction depends upon the degree of dehydroxylation of the surface of silica. On silica dehydroxylated at 500°C the reaction leads to one well-defined surface complex. On the other hand, on silica dehydroxylated at 200°C, the evolution of alkane is continuous. The difference in the latter case is related to the presence of neighboring OH groups, because the number of the surface vicinal OH groups capable of... 

Vicinal, where two silanol groups are close together on the surface and thus can interact by hydrogen-bonding... 

Isolated silanol group Vicinal silanol groups Geminal silanol groups... 

The siliceous surface provides various reactive sites that have been assigned to distinct silanol groups (isolated, perturbed isolated, vicinal, geminal) and strained... 

Figure 14.1 Silsesquioxanes used as models for Isolated, vicinal and geminal surface silanols.

To model the photochemical reaction of Re2(CO)io with silica in the presence of THE, silanolates other than silsesquioxanes [Et3SiOH, PhsSiOH, Ph2Si(OH)2 and (OH)Ph2SiOSiPh2(OH)j with [Re2(CO)8(THE)2] have also been synthesized [148], pointing to the formation of the surface species [((r-=SiO)((r-H)Re2(CO)8j in the above-mentioned reaction with silica. Rhenium carbonyl of the two diol derivatives listed above where both the SiOH have reacted are proposed as models for surface reaction with a vicinal hydroxyl residue, although no direct surface species is reported [148]. 

In addition, the infrared examination of the mechanism of propane and oxygen interaction with the sample (Fig. 6) indicates the different mechanism of interaction of the intermediate propylene as compared to other supported vanadium catalysts such as V-Ti02 (10). In particular, the formation of a 7t-bonded complex stabilized by a nearlying silanol with weak basic character due to the inductive effect of vicinal vanadium is shown. This indicates the relative inertness of the V sites in the silicalite towards 0-insertion or allylic H-abstraction on the adsorbed propylene. It is evident that the reduced reactivity of V sites in these reactions limits the consecutive reactions of intermediate propylene, thus enhancing the selectivity in the formation of this product. 

Silicagel is also called silica or bare silica. Its adsorptive properties depend on the hydroxyl groups attached to surface silicon atoms. Silicagel has a maximum silanol density of 8.0 yumolcs/ m2. Many of these silanols are buried deep in the porous structure and are available only to the smallest analytes. Silanols are either isolated, geminal, or vicinal they can be distinguished by means of Si solid-state nuclear magnetic resonance (NMR). The surface also contains siloxane bonds (Si-O-Si), which are considered hydrophobic. 

The hypothetical replacement of hydrogen by hydroxyl in silane, or in an alkyl or aiyl silane, results in a series of silanols, formally analogous with the alcohol, vicinal glycol, hydrated carboxylic acid, and ortho-carbonic acid series of carbon chemistry. The silanols are in many ways both the most interesting and the most important silicone intermediates. 

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