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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

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infrared spectroscopy

Infrared Spectroscopy

Dr. Milkevitch

Organic Chem II Lab

Spring 2010

Feb 11 & 13, 2010

  • 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
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
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
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
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
  • 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
spectroscopy is the study of the interaction of matter and electromagnetic radiation
Spectroscopy Is the Study of theInteraction of Matter andElectromagnetic Radiation
  • In Organic Chemistry, the common techniques include:
    • Infrared Spectroscopy
    • Nuclear Magnetic Resonance Spectroscopy
    • UV/Visible Spectroscopy
what spectroscopy tells us
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)
when ir radiation is applied to a molecule
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

E = hv = hc


What an IR Spectrum Looks Like
  • Graph of absorption intensity vs. radiation frequency
    • Given as % transmittance
    • Units are in wavenumbers (cm-1), (sometimes microns)
now organic molecules are quite diverse
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
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
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
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 2250 – 2100 cm-1

Alkene 1680 – 1600 cm-1

Alkane 1200 – 800 cm-1

trends carbon hydrogen stretching
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
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 1650cm-1
    • Where C=C stretching occurs
  • Many of the important functional classes are identified by the presence (or absence) of absorptions in these regions
ft ir spectrometer
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


  • Table:
  • Summary of
  • Notable IR
  • absorbances for
  • the functional
  • Groups
  • Very important
  • For the organic
  • Chemist


  • 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)?

      • 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.