1. Infrared Spectroscopy
Dr. Milkevitch
Organic Chem II Lab
Spring 2010
Feb 11 & 13, 2010

2. 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

3. 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

4. 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

5. 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

6. 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

7. Summary of Relationships

8. 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

9. The Electromagnetic Spectrum

10. 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

11. 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)

12. The Infrared Region

13. 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

14. What an IR Spectrum Looks Like
Graph of absorption intensity vs. radiation frequency
Given as % transmittance
Units are in wavenumbers (cm-1), (sometimes microns)
E = hv = hc λ

15. 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

16. Ways Molecules Vibrate: 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

17. 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?

18. 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 – 2100 cm-1
Alkene – 1600 cm-1
Alkane – 800 cm-1

19. 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

20. 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

21. An Infrared Spectrophotometer
=>

22. 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

23. An Alkane IR Spectrum: Notable Peaks (or absorbances)

24. An Alkene IR Spectrum: Notable Peaks

25. An Alkyne IR Spectrum: Notable Peaks
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26. Correlation Table: Summary of Notable IR absorbances for
the functional
Groups
Very important
For the organic
Chemist

27. 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

28. 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

29. 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

30. Conclusions: Things to think about
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?

31. Overall, what did you learn about IR spectroscopy?
Do the peaks correspond to what type of compound you have (alkane, alkene or alkyne)?
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.