8. Infrared Spectroscopy
Infrared (IR) Spectroscopy is an analytical method that measures the
absorbance of a select band of electromagnetic radiation by a sample. The
wavelength of radiation that is absorbed is characteristic of different types of
chemical bonds. The graphical representation of the absorption of the
electromagnetic radiation by the sample makes an IR spectrum.
Organic chemists use infrared spectroscopy as a means of identifying
various types of bonds within in a molecule which determine the functional
groups present or absent. This information is used to determine the structure
of compounds. IR is usually used in combination with other techniques,
especially nuclear magnetic resonance (NMR) spectroscopy.
In this class you will learn how to interpret an IR spectrum and how to
operate a Perkin-Elmer and/or Nicolet infrared spectrometer. During this
experiment you will obtain an IR spectrum of an unknown compound and use
this information to identify the functional groups present and select the
identification of the material from a list of possible materials.
PRE-EXPERIMENT ASSIGNMENT
Study this chapter of the manual, the lecture notes on the Chemistry
Department web site, the “Sample IR Spectra” posted on the web site and the
appropriate section in your lecture textbook. Complete the take home
assignment relating to Infrared. This will be due at the very beginning of the
lab period.
A major part of this lab will be in operating an Infrared Spectrometer.
Your instructor will explain and show you how to do this. Since all the steps
involve operation of equipment, a pre-lab notebook procedure does not need to
be written. It will however be very important for you to write down in your
notebook all of the possible unknown compounds from the on-line notes. If you
do not have this information, it will be very difficult for you to successfully
determine the identity of your unknown material and points will be lost. The
notebook however will not be graded. In lieu of these points, an in-class
problem set will be assigned.
A student who has prepared for the Infrared Spectroscopy experiment
should be able to:
1. Identify the relative energy, frequency, and wavelength of infrared light
compared with other types (including gamma rays, x rays, ultraviolet and
visible light, microwaves, and radio waves). Also, identify the change that
occurs in molecules when infrared light is absorbed.
2. Explain the relationship between frequency, energy, wavelength and
wavenumbers.
3. Define the major absorbance regions in the IR. Know the frequency which
each major functional group absorbs in the IR.
4. If given a functional group, choose in which of the 5 regions it will absorb.
5. Define, identify, recognize, and explain the use of each of the following:
IR (infrared) spectroscopy, neat samples, salt plates (including how to clean
them), Nujol mull, and KBr pellet.
6. Draw the structure given the name, or give the name from the structure, of
the compounds used in the day's experiment.
7. Identify and explain safety considerations for this experiment.
8. Perform the day's experiment safely and successfully.
Quizzes given after the experiment has been performed may also include:
9. Give or recognize a probable absorption frequency in cm-1 in the IR spectrum
of a compound containing any of the following functional groups; Amines,
Alcohols, Alkynes, Terminal Alkynes, Nitriles, Alkanes, Alkenes, Aldehydes,
Carboxylic Acids, Esters, Amides, Aldehydes, Ketones, and Ethers. (Detailed
instructions of spectrum interpretation can be found in “Sample IR Spectra”
located on the organic web site.
10. Draw a reasonable IR spectrum if given a compound name or structure.
11. Determine the presence (or absence) of functional groups in a sample from
infrared spectral data. The data may be given in the form of a list of peak
locations (see #6 above) or as a spectrum.
Safety Precautions
All of the compounds you will take IR spectra of in this experiment are at
least slightly toxic and flammable. Wash your hands after conducting the
experiment.
Infrared radiation is relatively (but not completely) harmless. The safety
instructions on the IR spectrometer say “Do not stare into beam,” because long
exposures to IR radiation can be damaging.
EXPERIMENT
Preparing the sample
Handle salt plates with care. The salt plates are very sensitive to
moisture and will break easily. Each pair costs approximately $25. If you are
the first student, remove the jar containing the salt plates from the desiccator.
(A desiccator is a low humidity vessel.) Remove two salt plates from cotton
envelope. Place the two salt plates on a Kim-Wipe. Clean plates by placing a
few drops of acetone on each salt plate, then wiping clean with Kim-Wipe.
Throw Kim-Wipe away. Place 1-2 drops of unknown liquid on one salt plate.
Cover with other salt plate. Carefully place in sample holder. Take sample to IR
to obtain spectrum.
Obtaining the IR spectrum
Take sample to IR spectrometer. Follow instructions posted adjacent to
machine.
Cleaning the salt plates
Remove salt plates from sample holder. Place salt plates on Kim-Wipe.
Place a few drops of acetone on each salt plate. Using a new Kim-Wipe, rub
each salt plate with the acetone. If another student is waiting to run the
spectrometer, help them go through the process. If you are the last user,
replace each salt plate in a separate cotton envelope. Place each envelope
within the plastic jar. Place the plastic container back in the desiccator.
Throw away any used Kim-Wipes.
Cap your unknown sample. Replace unknown sample in its designated spot
within the unknown sample container.
POST-EXPERIMENT ASSIGNMENT
Complete the datasheet and turn in before leaving class. Ensure your IR
spectrum is stapled to the datasheet. Turn in the white notebook pages from
your lab notebook. Staple multiple sheets together. Tear off rough edges.
Prepare for the IR portion of the next quiz. Remember that you may be
asked questions on anything you should have learned from assigned readings,
pre-experiment lecture, or doing the experiment.
Revised March 27, 2016 S. L. Weaver
Functional Groups
Position (cm-1)
O-H/ NH
2500-3500
(Nitrogen & Oxygen to Hydrogen)
-O-H
3200-3500
-N-H
3200-3500
-COOH
2500-3500
C-H
(Carbon to Hydrogen)
sp ≡C-H
sp2 =C-H
sp3 C-H
2800-3100, 3300
≡
(Triple Bond)
C≡N
C≡C-H
2100-2300
=
(Double Bond)
C=O
C=C
1620-1800
Fingerprint
-C-O-
<1600
1000-1300
3300
3000-3100
2800-3000
2200-2300
2100-2260
1650-1800
1620-1680
Shape
strong, rounded
medium, pointed
very broad
strong, very pointed
meduim, shoulder
multi-peaked
weak sharp
very weak sharp
strong
medium, sharp
limited value
use only when necessary
Complete Table of Main IR Frequencies
Wave number, cm-1
3200-3400
Functional Group
-O-H (alcohol)
Peak Description
Strong and broad
2500-3500
-O-H (carboxylic acid)
Very broad (over ~ 500 cm-1), often
looks like distorted baseline, can be
above 3000-1.
3200-3500
N-H
Doublet in case of NH2 group of a
primary amine or amide.
3300
≡C-H ( sp carbon-H, terminal
alkyne)
Usually sharp and strong
3000-3100
=C-H (alkene or arene)
Often weak, overlaps with C-H
alkane absorption. Looks like
shoulder on other peak.
2800-3000
C-H (sp3 carbon)
Strong, broad and multi-banded
2200-2500
C≡N
Medium intensity
2100-2260
C≡C (alkyne)
Medium intensity for terminal
alkynes, very weak for internal
alkynes.
1650-1800
C=O (carbonyl of amides,
ketones, aldehydes, carboxylic
acids and esters)
Very strong; lower frequency for
amides and when C=O is conjugated.
1620-1680
C=C (alkene, aromatic ring)
Check to see if you have a sp C-H
unsaturated stretch at >3000cm-1 (if
not, it is completely substituted)
~1600
-NH2 (bending 1o amines and
amides)
Only if you have corresponding N-H
peak at 3200-3500 cm-1 (otherwise
this peak may be mistaken for C=C).
1200
Ar-H
1000-1300
690 and 750
C-O
Phenyl group
Strong (look for =C-H and C=C first,
this area easy to misidentify)
Easy to misidentify.
Strong (look for =C-H and C=C first,
this area easy to misidentify)