Infrared Spectroscopy and Electromagnetic Radiation

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  • Created by: LBCW0502
  • Created on: 29-12-17 16:22
What is IR spectroscopy used to identify?
Functional groups in a compound
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What is the unit for the frequency in IR spectroscopy called?
Wavenumber (cm^-1)
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On the wavenumber scale, IR absorptions occur in which regions?
4000 - 400 cm^-1
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What happens to the energy as the frequency is increased?
As the frequency is increased, the energy is increased (wavenumber proportional to frequency)
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Absorption of IR light causes what type of changes in a molecule?
Changes in vibrational motions
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What are the two different vibrational modes in a molecule?
Stretching and bending (covalent bonds are not static)
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What are the labels on the axes on the IR spectrum?
Wavenumber on x-axis (cm^-1) and % transmittance on y-axis
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Where does the functional group region occur?
Above 1500 cm^-1
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Where does the fingerprint region occur?
Below 1500 cm^-1
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Where a particular bond absorbs IR depends on which two factors?
Bond strength (stronger bonds vibrate at a higher frequency) and atomic mass (lighter bonds vibrate at a higher frequency)
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Describe aspects of Hooke's law
Describes the motion of a vibrating spring. Force constant F (strength of bond). Mass m
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Bonds absorb at how many predictable regions in an IR spectrum?
4
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Absorptions for bonds to hydrogen occur on which side of the IR spectrum?
Left side (higher wavenumber region). H has little mass - vibrates at a higher frequency
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How do the absorptions for C=C to C-C move across the IR spectrum?
Moves father to the right of the IR spectrum. Bond strength decreases with frequency of vibrations
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How does the percent s-character affect bond strength and wavenumber of absorption?
The higher the percent s-character, the stronger the bond and the higher the wavenumber of absorption
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For a bond to absorb IR, is there a change in dipole moment during the vibration?
Yes
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Do symmetrical non-polar bonds absorb IR?
No (IR inactive) - stretching of bond axis does not change the dipole moment
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Where is the major absorption peak for sp3 hybridised carbon atoms?
2850 - 3000 cm^-1 (above 1500 cm^-1)
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Where is the major absorption peak for sp2 hybridised carbon atoms?
3000 - 3150 cm^-1
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Where is the major absorption peak for C=C?
1650 cm^-1
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Where is the major absorption peak for C-H with sp hybridisation?
3300 cm^-1
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Where is the major absorption peak for C-H with sp3 hybridisation?
2850 cm^-1 - 3000 cm^-1
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Where is the major absorption peak for C triple bond C?
~2250 cm^-1
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Where is the major absorption peak for OH?
3200 - 3600 cm^-1
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Where is the major absorption peak for C=O?
1700 cm^-1
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Where is the major absorption peak for N-H?
3300 - 3400 cm^-1
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Where is the major absorption peak for C=O in an amide?
1600 cm^-1
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Where is the major absorption peak for cyanide (in a nitrile)?
~2250 cm^-1
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What is wavelength?
The distance from one point on a wave to the same point on the adjacent wave
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What is frequency?
The number of waves passing a point per time (in cycles per second for Hertz)
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Is the speed to EM directly proportional to wavelength and frequency?
Yes, c = λv
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How was wavelength affect frequency?
The longer the wavelength, the lower the frequency. The shorter the wavelength, the higher the frequency
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Is the energy of photon directly proportional to its frequency?
Yes, E = hv
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What is Planck's constant?
6.63 x 10^-34 J.s (E increases, v increases but when E decreases, λ increases)
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Is the energy of electronic, vibrational and nuclear spins quantised or continuous?
Quantised
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What must happen for absorption to occur?
The energy of the photon must match the difference between two energy states in a molecule. The larger the energy between two states, the higher the energy of radiation needed for absorption (higher v and shorter λ)
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Other cards in this set

Card 2

Front

What is the unit for the frequency in IR spectroscopy called?

Back

Wavenumber (cm^-1)

Card 3

Front

On the wavenumber scale, IR absorptions occur in which regions?

Back

Preview of the front of card 3

Card 4

Front

What happens to the energy as the frequency is increased?

Back

Preview of the front of card 4

Card 5

Front

Absorption of IR light causes what type of changes in a molecule?

Back

Preview of the front of card 5
View more cards

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