Practice Interpretation of Spectra

One method of teaching interpretation of spectra to students is based on having them memorize a number of generalizations which, when applied to an unknown spectrum, lead the student to postulate several structures. To verify which of the postulated structures is correct, resort is then made to reference spectra or tabular data. 

The strongest bands in the spectrum should be assigned first. Usually their position and relative broadness suggest the class of compound to which the unknown belongs. 

Carbon-hydrogen absorption bands, in contrast to carbon-oxygen, carbon-nitrogen, carbon-sulfur, and most other carbon-atom bands, do not have a broad appearance. The converse is then true— i.e., broad bands usually are not carbon-hydrogen group frequencies. 

If these groups are present as XY2 units, then two bands are observed, one above and one below the position listed. 

The groups adjacent to the units listed in 4 can change the positions listed. For example, in CH3OH the C—O stretch is at 1050 cm. In CH3COOH it is at 1250 cm. The adjacent 

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Interpretation of the spectra of chemisorbed molecules presents some difficulties because the surface compounds formed during chemisorption have no exact counterparts among conventional compounds. Although some general principles can be applied, interpretations of the spectra of unknown species are usually based on empirical comparison with spectra of compounds of known structure. Experience has shown, however, that these difficulties are more philosophical than practical. Interpretations of spectra of chemisorbed molecules by comparison with the spectra of compounds of known structure have produced results which are self-consistent and reasonable in a wide range of applications. 

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