Infrared absorption characteristic frequencies

Acyl derivatives of cyclic hydroxamic acids, like all 0-acyI derivatives of hydroxylamine, show infrared absorption because of the acjd carbonyl at characteristically high frequencies, generally near 1800 The range of frequencies is similar to that found for... 

Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

The sparingly water-soluble triethylammonium saltofBi2H122-is readily converted to the highly water-soluble sodium salt. A carefully weighed sample of the triethylammonium salt is treated with an exactly equivalent amount of standardized aqueous sodium hydroxide. The mixture is heated to effect solution and then boiled to expel the free triethylamine until the vapors are no longer basic. Evaporation to dryness then leaves a crystalline hydrate of Na2Bi2Hi2. The anhydrous salt is obtained by drying at 100°/1 n for 24 hours and has characteristic infrared absorption frequencies as a Nujol mull (exclusive of any frequencies coincident with Nujol) at 2470 (s), 1010 (w), 1070 (m), 770 (w), 727 (m),710 (m),and 700 (m) cm. 1... 

The deep red colour and the characteristic low frequency infrared absorption (— 1600 cm-1) of the anion [Co3(CO)i0]" disappear simply by changing the solvent61. ... 

It is the di-cr species on Pt(lll) which is converted at higher temperature into ethylidyne. This conversion had also been investigated by infrared-visible sum-frequency generation (SFG) by Cremer et al. (371), a welcome first application of this new spectroscopic technique to hydrocarbon adsorption chemistry. They observed an absorption characteristic of an intermediate with a nCH3 band at 2957 cm-1 and suggested that this arises from an ethylidene M2CHCH3 (or possibly ethyl) species with its C-CH3 axis at an angle to the surface. It is very clear experimentally, as discussed in Part I, that ethylidyne on Pt(lll) is formed from di-cr-ethene and not directly from its 77-bonded isomer. 

Characteristic Infrared Absorption Frequencies of Carbonyl Compounds3... 

Describe the characteristic infrared absorption frequencies that would allow you to distinguish between the following pairs of compounds. 

Table 2.ia Characteristic infrarred absorption frequencies of common classes of organic compounds... 

Fig. 3.30 Schematic representation of infrared and Raman experiments. In infrared spectroscopy the excitations are detected by absorption of characteristic frequencies, in Raman spectroscopy the excitations are detected by characteristic s/ti/is in frequencies of the scattered light, [rrom Harris, D. C Bertolucci, M. D. Symmetry and Spectroscopy, Dover New York, 1989. Reproduced with permission.]...

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