Hydroxyl group characteristic vibration

The rotational mobility of adsorbed molecules is caused by its rotational degree of freedom (resulting from the fact that the molecule is tightly bound to the substrate through the only atom) and by the coupling of molecular vibrations with surface atomic vibrations. The rotational motion intensity is strongly temperature-dependent and affects spectroscopic characteristics. As a result, the rotational mobility of surface hydroxyl groups was reliably detected.200 203... 

The IR spectra of hydroxy adds are not very different from the spectra of the corresponding carboxylic acids. The additional hydroxyl groups usually produce minor displacements of the carbonyl vibrational bands to higher energies and, of course, contribute additional characteristic... 

As mentioned above, HOSi(OA)3 may be taken as the simplest cluster model of the terminal hydroxyl group in silicas. Indeed, even with this cluster CNDO/BW provided a quite satisfactory description of the lower part of the curve representing potential energy as a function of the OH stretching vibration coordinate ROH (Fig. 2) (48,49). The respective experimental curve was plotted by Kazansky et al. (49) based on the analysis of the fundamental frequency vOH and the first overtone of the characteristic OH stretching vibration in terms of the Morse potential function. The frequencies of the second and third overtones were also determined in that work, and it was shown that the Morse potential reproduced well the potential curve within a rather wide range of ROH. 

The hydroxyl group of serine and threonine should have a characteristic deformation mode, probably in the region 1350-1250 cm" in the IR (Bellamy, 1975). In j3-(Ser) , it has been assigned to a Raman band at 1399 cm" (Koenig and Sutton, 1971). Since this mode is likely to be mixed with other backbone vibrations, it is probably a poor group frequency. 

Upon adsorption of pyridine IR bands at 1540 and 1450 cm" appear characteristic for Bronsted and Lewis acid sites, respectively. The band at 1490 cm is due to the combination vibration of both types of acid sites. On pyridine adsorption the OH band typical of Bronsted acidic hydroxyl groups disappeared while those of the terminal SiOH groups only decreased in intensity (see Fig. 2). These observations reflect the different acidity of these OH groups. The parent Na-ZSM-5 sample possessed no Brdnsted acidity detectable by this method. 

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