Hydroxyl group, frequency shift surface

Precursors useful in the surface sol-gel process are not restricted to alkoxides. The requirements as precursors are chemisorption on surface hydroxyl groups and regeneration of the hydroxyl groups after hydrolysis. For example, TiO(acac)2 repeatedly adsorbs, when acid and alkali are added to the adsorption and hydrolysis media, respectively. Except for the case of Nb(0"Bu)5, all the compounds listed in Table 6.1 show linear frequency shifts. Adsorption conditions such as concentration, temperature, and immersion time are dependent on the solubility, reactivity, and the ease of hydrolysis of alkoxides. These conditions are varied as the structure of alkoxide units is changed. For example, Ti(01Pr)4, which exists as a monomer in solution [17], requires conditions different from Ti(0"Bu)4, which tends to form oligomer species. 

A clear correlation between isotropic H chemical shift and the frequency of the O-H stretching vibration has been reported [61] for surface hydroxyl groups in zeolites and related materials, as well as for water molecules in solid hydrates and strongly hydrogen bonded protons in inorganic solids. 

Several types of cells have been constructed and are commercially available that allow an in situ measurement or have a movable holder to evacuate and heat the sample. With these cells it is also possible to admit gas or vapor to subsequently measure the frequency shift upon adsorption. This technique can reveal additional information about the surface properties of the material. Carbon monoxide, for instance, has been applied as a probe to detect hydroxyl groups at the silica surface. Upon adsorption of CO at 77 K, the band due to free hydroxyl groups shifts to a lower frequency by 78 or 93 cm-1 (35, 36, 37). 

The frequencies assigned to isolated hydroxyl groups on surfaces are known to be shifted even as a result of the adsorption of nonpolar gases. McDonald (32) attempted to find a correlation between some physical property of the adsorbates and the magnitude of these frequency shifts, S 0H. He suggested that the polarizability of the adsorbate... 

It is generally accepted that hydroxyl groups of dehydrated silica surface are the main sites of adsorption of the molecules capable to form hydrogen bonds with these sites (water, alcohol, amines). This conclusion was made on the base of investigation of the shift of vibration bands of free Si—OH groups to low frequency region [49,50]. The following models are proposed, for example, for the adsorption complexes of water on silica surface ... 

CO-induced shift of the 0-H stretching modes, cm-i Figure 2.24 Correlation between the CO stretching frequency and the CO-induced shift of the 0-H stretching modes of surface hydroxyl groups. Data from Table 2.6, circles data from Table 2.7, stars data from Table 2.8, triangles and data from Table 2.9, squares. 

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