Reminder: These notes are meant to supplement, not replace, the laboratory manual.
Infrared Spectroscopy
History and Application:
Infrared (IR) radiation is simply one segment of the electromagnetic spectrum of
which visible light is a part. Human eyes do not detect infrared radiation. Infrared
spectroscopy is a method to quantify how much of this type of radiation is absorbed by
sample materials. Different chemical functional groups absorb specific wavelengths of
radiation in the IR range. By examining the characteristic absorbance patterns of a
compound, the functional groups present can be determined.
The first commercial Infrared Spectrometer was offered for sale by Perkin-Elmer
in 1944.i These early analyses were very labor intensive exercises. The first computer
controlled IR was also developed by Perkin-Elmer in 1976.i The XULA organic
chemistry labs have two types of Fourier Transform Infrared (FTIR) machines, PerkinElmer and Thermo Scientific. Each Perkin-Elmer machine cost approximately $25,000,
and the newer Thermo Scientific FTIR cost about $14,000. These machines obtain high
quality spectra rapidly and are easy to use.
IR spectroscopy is a reliable and facile analytical technique that has many real
world applications. IR is used in many food and chemical production facilities to
measure product qualityii ranging from measuring the fat and moisture in meat to film
thickness of polymer sheets.
Laboratory Application
1.
Here is a list of the types of electromagnetic radiation, in order of decreasing
energy per photon.
gamma rays > x-rays > ultraviolet > visible > infrared > microwaves > radio wave
In the equation E = ħ = ħ c/, E is the energy, ħ is Planck’s constant, is the
frequency, c is the velocity of light, and  (lambda) is the wavelength. The
velocity of light in a vacuum is a constant value (3.0x108 m/s). This means that
1
 ∝ ∝ 𝜆or that Energy is proportional to frequency and frequency is
proportional to the inverse of wavelength. In other words as energy increases,
frequency increases, and wavelength decreases.
2.
The most commonly used unit of frequency is the number of waves per second
(waves/second or s-1) which is also called hertz. Radio stations are commonly
referred to by the frequency of their signal. For instance WWNO operates at 88.9
MHz (88.9 mega hertz or 88.9 x 1*106 = 88,900,000 waves/second.) When
dealing with IR a different unit of frequency is used. IR uses wavenumbers which
is the number of waves per centimeter (waves/cm or cm-1) or reciprocal
centimeters as the standard frequency measurement. This simply describes the
number of waves of that particular electromagnetic radiation that will fit within one
centimeter. As the wavenumber increases, the wavelength decreases, and the
energy increases.
The image on the left has a higher
wavenumber (more waves per unit of distance), and therefore a shorter
wavelength and higher energy than the image on the right. A typical IR spectrum
spans 500 wavenumbers (cm-1) to 4000 wavenumbers (cm-1).
3.
Chemical bonds are formed by electrons being shared between two nuclei.
Different chemical bonds absorb different energy (different wavenumber) of
infrared radiation. The absorbed energy causes the bond to vibrate, stretch or
bend faster (change to a higher-energy state). The analysis of the different
characteristic IR energy being absorbed by a material allows the detection of
different chemical bonds within a molecule.
4.
IR spectroscopy is the study of molecules by their absorption of infrared light.
Chemists use IR spectroscopy to identify the functional groups present in
molecules. We also compare IR spectra of unknown compounds with literature
spectra of known compounds, for purposes of identification. These comparisons
are most often performed using computers. IR spectroscopy does not provide
complete structures of materials, it only provides information about the functional
groups present.
5.
Different types of samples can be used when taking IR spectra.
Neat samples are liquid samples with no added solvent. (We don’t call them
pure samples because they can be mixtures, and often are.)
Salt plates are disks or rectangles of clear salt (typically NaCl) with parallel faces.
Glass plates cannot be used because glass (SiO2), while transparent in
the visible range, is opaque to IR frequencies. Handle salt plates only by
the edges, because fingerprints can contaminate the plates. Do not allow
any water or wet solvents to contact the salt plates. Neat samples or Nujol
mulls are placed between two salt plates for IR spectroscopy. To clean
salt plates, rinse them with acetone or another dry organic solvent. DO
NOT rinse them with water or a solvent containing water (such has 95%
ethanol) or the salt will dissolve.
Nujol is a brand name of mineral oil similar to baby oil. Mineral oil is distilled from
petroleum, and is a mixture of alkanes having medium molecular weights
(higher than kerosene, lower than paraffin). A Nujol mull is a thick
suspension of a solid compound with the oil. Mulls are made by grinding
the solid and oil together in a mortar and pestle.
KBr pellets are solid mixtures of potassium bromide (a salt used to tread
epilepsyiii) and a solid sample. To make a KBr pellet mix a ground solid
sample with ground potassium bromide. Subject this mixture to high
pressure to form a disc shaped pellet. This pellet will be translucent or
transparent to IR light and an IR spectrum can be taken directly.
6. Each of you will receive an unknown compound. All of today’s unknown samples are
pure neat liquids. You will analyze your unknown in a FTIR spectrophotometer
as a thin film between two salt plates. An IR spectrum will be obtained. Your
careful analysis of the spectrum will allow you to correctly identify your unknown
material.
7. There are two different FTIR spectrophotometer instruments in the organic
instrumentation room (NCF 371). Spectrophotometer means any instrument
which measures the amount of light which passes through a material. The
Perkin-Elmer FTIR is pictured below on the left and the Thermo Scientific FTIR is
on the right.
8.
A typical IR spectrum is a graph. The X axis gives the frequency of the light in
reciprocal centimeters or wavenumbers. The Y axis gives the relative amount of
IR photons that are absorbed. If a sample does not absorb any IR photons, a
straight baseline will result close to 100 % transmittance ( 0 % absorbance). The
baseline is at the upper edge of the spectrum.
If a compound absorbs in the IR, a peak will result.
The compound which was responsible for the above spectrum has a number of
characteristic absorbancesiv. The strongest absorbance is at 1720 cm-1. You will soon
learn to see that this particular absorbance is characteristic of carbonyl (C=O) groups.
9. Below are the 12 possible unknown compounds and the material used to clean the
salt plates, acetone. There are no chemical reactions for this experiment. Write all of the
structures and names of the compounds in your notebook.
Propanoic Acid
(carboxylic acid)
Cyclohexanol
(alcohol)
Butyl Acetate
(ester)
2-butanone
(ketone)
1-Hexyne
(terminal alkyne)
2-methoxy-2-methyl propane (MTBE)
(ether)
acetonitrile
(nitrile)
cyclohexene
(alkene)
H2
C
H3C
H2
C
C
H2
H2
C
C
H2
CH3
Heptane
(alkane)
1,6-Diaminohexane
(primary amine)
-Butyrolactam
(a cyclic amide)
2-Propanone (Acetone)-Used to clean salt plates,
not a possible unknown(ketone)
10.
Safety considerations:
All of the compounds you will take IR spectra of in this experiment are flammable
and slightly toxic. 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 can be damaging. Normal IR exposure is not hazardous.
11.
Read over and print out a paper copy of “IR Spectra Interpretation” found on the
2230L Organic Chemistry I Lab web site. Bring this with you to lab. Your
instructor will go over methods to properly and easily interpret IR spectra. This
information will be used to determine the identity of your unknown material.
REFERENCES
i
Perkin-Elmer History of IR
http://las.perkinelmer.com/content/RelatedMaterials/Brochures/BRO_60YearsInfraredSpectroscopy.pdf (December
26, 2010)
ii
NDC-Online Measurement Company description of applications http://www.ndcinfrared.com/NDC/ (September 7,
2011)
iii
Food and Drug Administration http://www.fda.gov/AnimalVeterinary/ResourcesforYou/ucm301671.htm
(September 4, 2015)
iv
Spectra Database, SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced
Industrial Science and Technology), (December 26, 2010)
Revised September 4, 2015 S. L. Weaver