10.6: Information Obtained from IR Spectra

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• IR is most useful in providing information about the presence or absence of specific functional groups.

• IR can provide a molecular fingerprint that can be used when comparing samples. If two pure samples display the same IR spectrum it can be argued that they are the same compound.

• IR does not provide detailed information or proof of molecular formula or structure. It provides information on molecular fragments, specifically functional groups.

• Therefore it is very limited in scope, and must be used in conjunction with other techniques to provide a more complete picture of the molecular structure

IR ABSORPTION RANGE

The typical IR absorption range for covalent bonds is 600 - 4000 cm-1. The graph shows the regions of the spectrum where the following types of bonds normally absorb. For example a sharp band around 2200-2400 cm-1 would indicate the possible presence of a C-N or a C-C triple bond.

THE FINGERPRINT REGION

Although the entire IR spectrum can be used as a fingerprint for the purposes of comparing molecules, the 600 - 1400 cm^-1 range is called the fingerprint region. This is normally a complex area showing many bands, frequently overlapping each other. This complexity limits its use to that of a fingerprint, and should be ignored by beginners when analyzing the spectrum. As a student, you should focus your analysis on the rest of the spectrum, that is the region to the left of 1400 cm^-1.