Infrared Spectroscopy

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# Infrared Spectroscopy - PowerPoint PPT Presentation

Infrared Spectroscopy. Dr. Milkevitch Organic Chem II Lab Spring 2010 Feb 11 &amp; 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

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
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
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
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
• 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
• 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

E = hv = hc

λ

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
• 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 2250 – 2100 cm-1

Alkene 1680 – 1600 cm-1

Alkane 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 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
• “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

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 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

• 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?