Vibrational frequencies free surface hydroxyls

Table 4.5. Vibrational frequencies of free surface hydroxyls on oxides with different crystal structures.

Table 2.2 Vibrational frequencies of free surface hydroxyls on oxides with different crystal structures (27,28) . OH frequency, cm Ownpn rnordinatinn ...

The most commonly used mode for characterization of surface hydroxyls is the stretching frequency, v(OH). The free OH ion vibrates at 3555.6 cm and its bonding to one or more atoms leads to an increase of this value up to 3800 cm The shift depends on the electrostatic field experienced by the OH group and on the ionic or covalent character of the bond. As a result, v(OH) is not specific and cannot be used for unambiguous identification of the nature of the hydroxyl. Some specificity is observed with covalently bound hydroxyls such as Si—OH and P—OH. 

The direct involvement of the surface hydroxyls in adsorption of a large variety of gas molecules on silica has been studied extensively by using IR spectroscopy (Hair, 1967 Kiselev and Lygin, 1975). Upon adsorption, the stretching vibration frequency of the free OH groups (at 3750 cm ) is shifted toward lower... 

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 ... 

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