Structure & Reactivity
IR9. Misleading Peaks
There are some practical problems that can make IR
interpretation in real life more difficult. Being aware of these problems may
make you double-check your suspicions:
and could be confused with an O-H peak.
- water in the sample. Since water contains O-H bonds, water in a sample
will make it appear as if the compound contains O-H bonds. There are
experimental techniques for removing water from a sample but they must be
- overtones in a spectrum. Overtones are absorptions occurring at
different multiples of the normal frequency. Strong overtones of carbonyl
peaks often occur at about twice the normal wavenumber. A large peak at 1750
cm-1 might be accompanied by a smaller peak at 3500 cm
In addition, there are complications that you may run into based on the
instrument or technique used to obtain the spectrum.
between 4000 and 2000
cm-1 and every 500
below that. Spectra from different
instruments may display this scale differently.
Different techniques may lead to increased absorption at one end of the
spectrum vs. the other. For example, an OH stretch is much more
dominant if the sample is prepared in a KBr mull than if the spectrum is
obtained via attenuated total reflectance.
- Note that the x-axis scale on the spectrum is not uniform: the scale is
indexed every 1000 cm
This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's
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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.
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