Infrared Spectroscopy
Dr. Milkevitch
Organic Chem II Lab
Spring 2010
Feb 11 & 13, 2010
Introduction
 The purpose of this experiment
 To introduce the student to
spectroscopy
 Discuss the specific technique of
Infrared Spectroscopy
 Which is used to acquire structural
information on organic molecules
 Use this technique in the laboratory
First: In order to Understand
Spectroscopy
 Must understand electromagnetic radiation
(EMR)
 EMR is a form of energy
 Has a particle and wave nature
 Examples: Light, microwaves, radiowaves
We Use Symbols to Designate
Properties of Waves
 λ is the wavelength of the waves
 ν is the frequency of the waves
 c is the speed of light
 of all EMR actually
Relationships Between These
Variables
 Speed = wavelength x frequency
 Therefore:
 c = λν
 λ = c/ν
 ν = c/λ
 For electromagnetic waves, the speed
(c) is constant
 3 x 108 m/s
What This Means
 Wavelength has a direct, inverse
relationship with frequency:
 λ ∝ 1/ν
 The higher the frequency, the shorter the
wavelength
 The longer the wavelength, the smaller
the frequency
Summary of Relationships
Wavenumbers
 When the wavelength is measured in
centimeters:
 the reciprocal of the wavelength (1/cm) Is
directly proportional to the frequency
 1/cm Is called the wavenumber and is a
commonly used term in spectroscopy
The Electromagnetic Spectrum
Spectroscopy Is the Study of the
Interaction of Matter and
Electromagnetic Radiation
 In Organic Chemistry, the common techniques
include:
 Infrared Spectroscopy
 Nuclear Magnetic Resonance Spectroscopy
 UV/Visible Spectroscopy
What Spectroscopy Tells Us
 Specific information on the structural features of
the molecules being studied
 The presence or absence of specific patterns of
chemical bonding in a molecule
 Infrared Spectroscopy: The use of infrared
radiation to determine the presence or absence
of specific patterns of bonding in a molecule
(i.e., functional groups)
The Infrared Region
When IR Radiation is Applied to a
Molecule

Some passes through it, but some does not

Some of it is absorbed

All bonds in a molecule have a vibrational frequency

If the frequency of the IR energy matches the specific vibrational
frequency of a bond in a molecule
 The molecule will absorb the IR radiation at that frequency

The bond is excited from a lower to a higher vibrational state
 Amplitude of vibration increases dramatically

We can measure this absorbance of IR radiation

We can come up with a graph of absorbance intensity vs.
Wavelength
What an IR Spectrum Looks Like
 Graph of absorption intensity vs. radiation frequency
E = hv = hc
Given as % transmittance
λ
Units are in wavenumbers (cm-1), (sometimes microns)
Now, Organic Molecules are Quite Diverse
 Millions of organic compounds exist
 Remember the 12 families of organic compounds?
 Structurally different molecules can have different functional groups
 Do not absorb exactly the same frequencies of IR radiation
 Therefore, give different patterns of absorption
 Specific bonds and functional groups in a molecule
 Have specific vibrational frequencies
 Therefore, will absorb characteristic frequency ranges of IR
radiation
 This means:
 IR spectroscopy is a valuable tool for identifying different
functional groups
 Also, a valuable tool for helping identify the structure of an
organic compound
Ways Molecules Vibrate:
Vibrational Modes
 Vibrational Modes:
 Fancy way to describe the ways a molecule can vibrate
 2 most important vibrational modes in IR spectroscopy:
 Stretching: involves a change in interatomic distance
 Bending: involves a change in bond angles
Change in interatomic distance
Change in bond angles
IR-Active and Inactive Bonds
 Stretching and bending must:
 Change the molecule’s dipole moment in order
to be IR active
 Large changes in dipole moment: very intense IR
absorption
 This is really important
 Polar bonds will absorb strongly
 Does a polar bond have a dipole moment?
 A nonpolar bond will absorb weakly or not at all
 Does a nonpolar bond have a dipole moment?
Some Trends in Vibrational Frequency
 The smaller the atoms in a bond, frequency
increases
 Larger the atoms in a bond, frequency
decreases
 Bond strength also effects frequency of
absorption
 Stronger bonds, higher frequency of absorption
Alkyne
Alkene
Alkane
2250 – 2100 cm-1
1680 – 1600 cm-1
1200 – 800 cm-1
Trends: Carbon-Hydrogen Stretching
 Bonds with more s character absorb at a
higher frequency
 More s character, shorter and stronger
bond
 sp3 C-H, just below 3000 cm-1 (to the right)
 sp2 C-H, just above 3000 cm-1 (to the left)
 sp C-H, at 3300 cm-1
The Three Most Important
Regions of the IR Spectrum
 3600 – 3100 cm-1
 Where OH and NH stretching occur
 Region around 1700 cm-1
 Where C=O stretching occurs
 Region around 1650 cm-1
 Where C=C stretching occurs
 Many of the important functional classes are
identified by the presence (or absence) of
absorptions in these regions
An Infrared Spectrophotometer
=>
FT-IR Spectrometer
 “The modern IR spectrometer”
 Small and compact
 Computer controlled
 Has better sensitivity than dispersive instruments
 Irradiate the sample with all IR frequencies at the same
time
 Does multiple scans quickly
 Averages the results
An Alkane IR Spectrum: Notable Peaks
(or absorbances)
An Alkene IR Spectrum: Notable Peaks
An Alkyne IR Spectrum: Notable
Peaks
=>
Correlation
Table:
Summary of
Notable IR
absorbances for
the functional
Groups
Very important
For the organic
Chemist
Procedure
Each group should choose one of the following compounds:
Chlorooctane
Dodecane
Chlorodecane
Cyclohexane
Cyclohexene
3,3-dimethyl-1-butene
Heptane
Hexane
1-heptyne
1-hexene
Procedure (2)

Obtain an IR spectrum


See Dr. M in the instrument lab
Interpret major absorption frequencies


Using tables in this handout
Annotate the spectrum with your
interpretations
Your Report



Your introduction should include a discussion of IR
spectroscopy
 Your textbook also has a chapter on IR
Spectroscopy, use it if necessary
No reaction mechanism or balanced equation in this
experiment
Physical properties section should be the relevant
physical properties of your chosen compound

Your results section should include the spectrum of
your chosen compound

Annotated




What do the peaks correspond to?
If it’s not annotated, it is meaningless
Make a table of relevant absorbances, along with their identity
Conclusions: Things to think about

Did you successfully obtain your IR spectrum?


Is the spectrum clean? Crappy? Easy to obtain?
What does your IR spectrum look like?


What major peaks do you have?
What stretches do these peaks correspond to?

Do the peaks correspond to what type of compound you have
(alkane, alkene or alkyne)?




1.
2.
3.
Prove this: correlate your peaks to what peaks these compounds
should have in an IR spectrum
Prove it further: find an IR spectrum of the compound and compare
it to your spectrum
Overall, what did you learn about IR spectroscopy?
Additional Questions to answer:
Which absorbs at a higher frequency: a C-H bond or a C-D
bond? Explain.
Why does H2 not have an IR spectrum?
Explain why the C=C stretch for a trans-disubstituted alkene is
weaker than for a cis-disubstituted alkene.