Infrared and Raman Spectra of Organic Compounds

Interpretation of Infrared and Raman Spectra of Organic Compounds 

A complex molecule can be considered as a system of coupled inharmonic oscillators. Empir- 

In the figures in this chapter (Figs. 8-44), the upper curve is the infrared spectrum, with intensity increasing from the bottom to the top of the diagram. The lower curve is the Raman spectrum with an ordinate linear in relative intensity units increasing from the bottom to the top of the diagram. The Raman spectra have not been corrected for fluctuations in the sensitivity of the spectrometer. The infrared spectra were recorded with a Bruker IFS 66 FT-IR spectrometer for recording of the Raman spectra a Bruker FRA 106 FT-Raman accessory was used. 

The CH2 wagging bands are spread over a region between 1350 and 1180 cm , occurring as a characteristic progression of weak bands. They are best seen in the solid-phase spectra of long straight-chain compounds such as fatty acids [10). 

The CH2 twisting vibrations in CHy chains have frequencies dispersed over the same region as the wagging bands, as can be seen in the spectra of polyethylene [25], for example. The infrared intensity of these bands is weak, whereas in the Raman spectrum at about 1300 cm the in-phase CH2 twist vibration is a useful group frequency. 

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Szymanski, H.A. (1989). Correlation of Infrared and Raman Spectra of Organic Compounds. Hertillon Press. 

H. A. Szymanski, Infrared Band Hand Book, Vols I-[II, Plenum, New York, 1964, 1966, 1967, and Correlation of Infrared and Raman Spectra of Organic Compounds, Hertillon, 1969. 

Nyquist, R. A., Kagel, R. O., Putzig, C. L, Leugers, M. A., Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts (4 vols.). Academic Press, New York and London, 1996. 

The Aldrich Library of Infrared Spectra, 3rd ed., Aldrich chemical Company, Milwaukee, Wl, 1981, by Pouchert contains >12,000 IR spectra so arranged that the user can readily see the change that takes place in a given spectrum when a slight change is made in the structure of a molecule. The same company also publishes the Aldrich Library of FT-IR Spectra and the Aldrich Library of NMR Spectra, both also by Pouchert. A similar volume, which has IR and Raman spectra of 1000 compounds, is Raman/Infrared Atlas of Organic Compounds, 2nd ed., VCH, New York, 1989, by Schrader. 

In this section of the present book a general survey of characteristic vibrations in organic molecules is given. By a specific discussion of typical features of the characteristic vibrations of distinct groups and of typical classes of compounds, their inherent dependence on bond properties, symmetry, structural characteristics and their environment is demonstrated. Since infrared and Raman spectra supply complementary images of a vibration, both spectra are discussed simultaneously and combined infrared and Raman spectra are reproduced as examples. They are taken from the atlas published by Schrader (1989). 

Extensive compilations of IR and Raman frequencies are available, in some cases with FTIR and Raman spectra plotted together for comparison (6-9). A few frequencies for organic compounds are listed in Table 2.1, in part to illustrate differences in IR and Raman intensities. Symmetric vibrations such as the acetylenic C — C stretch, the —S — S— stretch, and ring breathing modes are generally strong in the Raman but forbidden in the infrared, while... 

One advantage of studying organic compounds is that it is usually possible to obtain infrared, Raman and INS spectra of the material and this should always be the aim. The complementarity of the methods has been stressed previously and is evident from Fig. 8.1 modes that are strong in one form of spectroscopy are weak or absent in the others. 

It is assumed that the reader has some basic understanding of spectroscopy and organic chemistry. For a further understanding of underlying theories of infrared and Raman spectroscopy, the reader is directed to standard reference texts on the structure/spectra relationships of molecular compounds [90-94]. 

RA Nyquist, CL Putzig, MA Leugers. Infrared and Raman Spectral Altas of Inorganic Compounds and Organic Salts Raman Spectra. San Diego, CA Academic Press, 1997. 

J. B. Schrader, Raman/Infrared Atlas of Organic Compounds, 2nd ed., VCH Publishers, New York, 1989. 1118 pp. Raman and IR spectra plotted on the same scale for about 1000 compounds. The spectra are large they are vertical on the page, and each page has one pair of spectra. Very good. 

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