Most spectra using electromagnetic radiation are presented with wavelength as the X-axis. Originally, IR spectra were presented in units of micrometers. Unfortunately, a linear axis in micrometers compresses the region of the spectrum 10-15 mm) that usually has the largest number of peaks.
One could rectify this by presenting the speectrum on a linear scale vs. frequency (Hz), but the magnitude is unwieldy (10 mm = 3 x 1013 Hz). A different measure, the wavenumber, is given the unit cm-1. See "A Word about Units" below.
Once you collect a spectrum, the real work begins. Spectra of organic compounds have
two general areas:
|4000-1500 cm-1||1500-400 cm-1|
|The Functional Group Region
Peaks in this region are characteristic of specific kinds of bonds, and therefore can be used to identify whether a specific functional group is present.
|The Fingerprint Region
Peaks in this region arise from complex deformations of the molecule. They
may be characteristic of molecular symmetry, or combination bands arising from multiple
bonds deforming simultaneously.
word about units. Most spectra using electromagnetic radiation are presented
with wavelength as the X-axis. Originally, IR spectra were presented in units of
micrometers. Unfortunately, a linear axis in micrometers compresses the region of
the spectrum 10-15 mm) that usually has the largest number of
peaks. One could rectify this by presenting the spectrum on a linear scale vs.
frequency (Hz), but the magnitude is unwieldy (10 mm = 3 x 1013
Hz). A different measure, the wavenumber, is given the unit cm-1.
The relationship can be derived by the relationship
| The two regions of the spectrum overlap to a degree.
(In fact, one always finds overlap between different regions of any spectrum; these
designations are "guideposts" to help you orient yourself.) For example,
carbon-chlorine bonds appear at around 800 cm-1, and C-O single bonds appear at
around 1200-1300 cm-1. Also, benzene rings show "overtones" in
the 1500-1700 cm-1 region, even though these arise from complex ring
The normal way to approach interpretation of an IR spectrum is to examine the functional group region to determine which groups might be present, then to note any unusually strong bands or particularly prominent patterns in the fingerprint region. Finally, if you think you have identified the compound (usually you need additional information) you can compare the spectrum with a reference. Matching the fingerprint region is a very rigorous test.
Some important IR-active functional groups, and examples of spectra.
|Group||Region||Examples of spectra. (Try to find the characteristic peaks.)|
|C-H||3000-3100 cm-1 (sp2)
2800-3000 cm-1 (sp3)
Monomeric forms: sharp.
H-bonding leads to broadening.
Very broad, medium intensity
Usually weak; maybe not visible
in internal alkynes.
Nitriles are quite strong.
Often difficult to assign,
depending on fingerprint region.
Usually sharper than O-H.
A final word about symmetry.
Molecular vibrations give rise to IR bands only if they cause a change in the dipole moment of the molecule. (This comes out of the quantum mechanics of molecular absorption of energy, so we aren't concerned too much with why, yet.) If a stretch does not change the dipole moment, there won't be any IR band. This is why O2 and N2 in the atmosphere don't show any IR bands. CO2, however, has a stretch where one O moves in and the other moves out:
Thus we see this band at 2300 cm-1.
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Sample prepapration and use of the spectrometers.
Last updated: 01/26/13
Comments to K. Gable