An important tool of the organic chemist is Infrared Spectroscopy, or "IR". IR spectra are acquired on a special instrument, called an IR spectrometer. IR is used to gather information about compound's structure, assess its purity, and sometimes to identify it.
Infrared radiation is that part of the electromagnetic spectrum between the visible and microwave regions. Infrared radiation is absorbed by organic molecules and converted into energy of molecular vibration, either stretching or bending. Different types of bonds, and thus different functional groups, absorb infrared radiation of different wavelengths. A IR spectrum is a plot of wavenumber (X-axis) vs percent transmittance (Y-axis). (Note: wavelength can be used instead of wavenumber and absorbance instead of percent transmittance; please see the IR Spectroscopy Theory pdf file, referenced below.)
The Handbook for Organic Chemistry Lab (produced by the Chem Dept at CU Boulder) covers the theory of IR in depth. If you are interested, you can view this chapter as a pdf file:
This web-tutorial focuses on the band-positions in the IR spectrum of common organic functional groups, illustrating each with example spectra. Complex IR spectra are interpreted by extrapolating from such studies of simple molecules, since most functional groups give rise to bands in particular regions of the spectrum. The region from 4000-1300 cm-1 is particularly useful for identifying the presence of specific functional groups, although functional group bands in the region below 1300 cm-1 are also covered in this tutorial. The frame at the left lists the functional groups discussed. The last part of the tutorial explains how to approach the analysis of a spectrum of an unknown organic compound.
IR spectra are also analyzed by comparing observed spectra with spectra of known compounds. The fingerprint region to the right of 1300 cm-1 is particularly useful in this type of compound identification. To use this method, you need spectra of known compounds for comparison. In the organic chemistry teaching labs at CU Boulder, the spectra of all reactants and products are included in the appendices of the Handbook for Organic Chemistry Lab. Large compilations of printed IR spectra are available in the Science Library (see the librarian for help in finding these volumes; online versions might also be available). The spectra of most of the compounds you will use or make in the teaching labs are in the directories on the IR computers in the instrument room. This is useful because after you run your spectrum, you can overlay it with the spectrum of the compound that you think it is. Or, if you have an unknown, you can run a search of the database for a match with your compound.
Note: This tutorial covers the bands and the functional groups that are especially important to the organic chemistry student taking sophomore-level organic chemistry for pre-professionals. A lot more information is available for individual functional groups and for compounds with more than one functional group, as well as for groups not covered herein. For advanced students and for researchers, the stretching and vibrational frequencies of organic molecules are covered in depth in the literature references linked in the left frame.
Note: The IR spectra used in this tutorial were run on the Nicolet FT-IR instruments in the organic chemistry teaching labs.
Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.
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