Chem 150 – GRCC – K. Marr
FTIR Spectroscopy (Fourier Transform Infrared)
Acknowledgement: This document is adapted from a similar document produced by Dr. Kevin P. Gable, professor of Organic
Chemistry at Oregon State University. Used with permission.
Spectral Interpretation
Once you collect a spectrum, the real work begins. Spectra of organic compounds have two general areas:
4000-1500 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.
1500-400 cm-1
The Fingerprint
Region
A word about units. Most spectra using
Peaks in this region
are difficult to
identify, but each
organic compound
produces a distinct
spectrum within the
fingerprint region.
This region arises
from complex
deformations of the
molecule. They may
be characteristic of
molecular symmetry,
or combination bands
arising from multiple
bonds deforming
simultaneously.
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 m)
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 m = 3 x 1013 Hz). A different
measure, the wavenumber, is given the unit cm-1.
The relationship can be derived by the relationship
 (cm -1)= 10,000/(m)
The Functional Group and Fingerprint 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 deformations.
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.
Page 1 of 4
Chem 150 – GRCC – K. Marr
Some important IR-active functional groups and examples of spectra
Group Region
Examples of spectra. (Try to find the characteristic peaks.)
3000-3100 cm-1
(sp2)
C-H
2800-3000
cm-1 (sp3)
1600-1800 cm-1
C=O
Acids: 16501700
Esters: 17401750
Aldehydes:
1720-1750
Ketones:
1720-1750
Amides:16501715
3300-3600 cm-1
Monomeric
forms: sharp.
O-H
(alcohol) H-bonding
between
molecules
leads to
broadening.
Page 2 of 4
Chem 150 – GRCC – K. Marr
O-H
(acids)
CC
C
C-O
2400-3000 cm-1
Very broad,
medium
intensity
2200-2100 cm-1
Usually weak;
maybe not
visible
in internal
alkynes.
Nitriles are
quite strong.
1200-1300 cm-1
Often difficult
to assign,
depending on
fingerprint
region.
Page 3 of 4
Chem 150 – GRCC – K. Marr
N-H
3400 cm-1
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 2400 cm-1.
Page 4 of 4