Hydroxyl group vibrations

Table 12.2 Assignment of hydroxyl group vibrations for alumina from different models.

Heilweil E J, Casassa M P, Cavanagh R R and Stephenson J C 1984 Piooseoond vibrational energy relaxation of surfaoe hydroxyl groups on oolloidal silioa J. Chem. Phys. 81 2856-8... 

Section 15 14 The hydroxyl group of an alcohol has its O—H and C—O stretching vibrations at 3200-3650 and 1025-1200 cm respectively The chemical shift of the proton of an O—H group is variable (8 1-5) and depends on concentration temperature and solvent Oxygen deshields both the proton and the carbon of an H—C—O unit Typical... 

The vibrational overtones and combinations of hydroxyl groups and thek associated molecular water occurring in the spectra of various gel siUca materials are summarized in Table 2 and discussed in References 3, 5, and 22. These peaks and bands found in the preparation of alkoxide-derived siUca gel monoliths are identical to those described for siUca gel powders (41). 

Most methods of testing bond type involve the motion of nuclei. The chemical method, such as substitution at positions adjacent to a hydroxyl group in testing for double-bond character, as used in the Mills-Nixon studies, is one of these. This method gives only the resultant bond type over the period required for the reaction to take place. Since this period is much longer than that of ordinary electronic resonance, the chemical method cannot be used in general to test for the constituent structures of a resonating molecule. Only in case that the resonance frequency is very small (less than the frequencies of nuclear vibration) can the usual methods be applied to test for the constituent structures and in this case the boundary between resonance and tautomerism is approached or passed. 

Rising of CO partial pressure resulted in the appearance of a series of absorption bands with v (CO) 2176, 2156 and 2136 cm" in the area of CO stretching vibrations. The band at v (CO) 2176 cm can be attributed to the interaction of CO with weak Bronsted acid sites while the band at v (CO) 2156 cm can be attributed to the complex of CO with non-acid hydroxyl groups. The band with wavenumber v (CO) 2136 cm" can be related to physical-adsorbed CO over Silica. 

The broadband at 3270 cm-1 is due to the O-H stretching vibration of the hydroxyl group. Moreover, the N-H stretching vibration absorption for open-chain amides occurs near 3270 cm-1 in the niclosamide solid state. 

The bands at 1410 and 1210 cm-1 are due to combined C-O stretching vibration and O-H in-plane deformation of hydroxyl group of niclosamide [22]. 

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... 

Ryason and Russel measured the temperature dependence of the IR absorption band halfwidth for valence vibrations of hydroxyl groups on the silica surface.200 At T > 325 K, the least squares method permits a straight line to be drawn through experimental points of the dependence In Avv2 (Tl), the equation of the line appearing as follows 200... 

Figure 8.9 Diffuse reflectance infrared spectrum of a silica support, showing silica vibrations at frequencies below 1300 cm1, overtones and combination bands between 1700 and 2050 cm-1, and various hydroxyl groups at frequencies above 3000 cm 1. The sharp peak at 3740 cm"1 is due to isolated OH groups, the band around 3550 cm 1 to paired, H-bonded OH groups, and the band around 3660 cm 1 to hydroxyls inside the silica (courtesy of R.M. van Hardeveld, Eindhoven).

Note that in all the examples discussed so far, infrared spectroscopy gives its information on the catalyst in an indirect way, via hydroxyl groups on the support, or via the adsorption of probe molecules such as CO and NO. The reason why it is often difficult to measure the metal-oxide or metal-sulfide vibrations of the catalytically active phase in transmission infrared spectroscopy is that the frequencies are well below 1000 cm-1, where measurements are difficult because of absorption by the support. Infrared emission and Raman spectroscopy, discussed later on in this chapter, offer better opportunities in this respect. 

Ionization of the phenol hydroxyl group in tyrosine shifts the 277-nm absorption peak to 294 nm and the 223-nm peak to 240 nm. The molar extinction coefficient for the peak of the lower energy band increases from about 1350 M l cm-1 to about 2350 M em-1 and for the higher energy band from about 8200 M cm-1 to about. 1,000 M l cm-1.113 141 In addition, the lower energy absorption band of tyrosine shows vibrational structure that is lost upon ionization of the phenol side chain. 

Using infrared spectroscopy, Yates (299) proved the existence of hydroxyl groups on anatase as well as on rutile. Both forms still contained some adsorbed molecular water after evacuation at 150°, as evidenced by the bending vibration at 1605 cm b After outgassing at 350°, no free water was detected. There remained two OH stretching absorptions in the case of anatase (at 3715 and 3675 cm ) and one weak band at 3680 cm with rutile. This is indication of the existence of two different types of OH groups on anatase. These results were confirmed by Smith (300). 

The above interpretation can also explain the IR spectra of the HYUS sanples. Indeed, the HYUS-8 sample shows two bands at 3555 and 3635 cm, a considerable amount of silanol groups, and hydroxyl vibrating at 3675 and--3695 cm (Fijg. 4a). After pyridine is adsorbed the bands at 3555 and 3635 cm disappear, while the other two bands remain and a new band at 3615 cm is now clearly visible (Fig. 4b. If the difference spectrum is obtained a band at 3600-3610 cm (Fig. 4c) again indicates the presence of strongly acid hydroxyl groups. ... 

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