Hydroxyl group Raman frequencies

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. 

Some years ago when studying the low frequency region in the Raman spectra of crystals and liquids we paid attention to some weak bands situated in the region of 150-300 cm-1 which appeared with various substances containing the hydroxyl group [1], With different substances these bands have different width and intensities. In Table 1 some examples are given and the approximate positions of the bands are listed. Such a band of low frequency is particularly strong in the spectrum of formic acid (Fig. 1). 

By comparisons among the spectra of large numbers of compounds of known structure, it lias been possible to recognize, at specific positions in the spectrum, bands which can be identified as characteristic group frequencies associated with the presence of localized units of molecular structure in the molecule, such as methyl, carbonyl, or hydroxyl groups. Many of these group frequencies differ in the Raman and infrared spectra. 

If the reacting molecules are already in contact, the rate is expected to be similar to that found for the recombination of H30+ and OH in ice, i.e. similar to the rate of vibration of a proton in a hydroxyl group. Under certain specialised conditions, the rates of recombination of H30 + and A in contact may be calculated from the width of lines in the Raman spectrum of a mixture of the acid and its conjugate base [15]. The trifluoracetate ion has a fairly narrow Raman peak at 1435 cm 1, attributed to the symmetrical carboxylate stretching frequency, which does not appear in acid solutions of trifluoracetic acid. Trifluoracetate— trifluoracetic acid buffers show a lower, wider peak than that for the trifluoracetate ion, and the width of this peak is inversely proportional to the time a proton spends on the carboxylate group. The variation of the width of the line with acid and anion concentrations gives a measure of the rate of transfer of a proton from an adjacent H30 + to the anion, and a rate coefficient of about 101 2 lmole 1 sec 1 has been calculated. 

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. 

Nakayama, S., F. Tani, M. Matsu-ura, and Y. Naruta (2002). Cobalt single-coronet porphyrin bearing hydroxyl groups in its 0-2 binding site as a new model for myoglobin and hemoglobin Observation of unusually low frequency of V(O-O) in resonance raman spectrum. Chem. Lett. 496 97. 

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