1 CHAPTER 9 Spectroscopy: the study of the interaction of energy with matter Energy applied to...
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CHAPTER 9
Spectroscopy: the study of the interaction of energy with matter
• Energy applied to matter can be absorbed, emitted, cause a chemical change, or be transmitted
• Spectroscopy can be used to elucidate the structure of a molecule
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Electromagnetic radiation
• Electromagnetic radiation is the energy that is transmitted through space in the form of waves.
• Types of electromagnetic radiation: Radio waves, Ultraviolet (UV), Infrared(IR), Visible (vis)..
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Characterization of waves• Waves are characterized by:
1- Wavelength (λ): the distance from the crest of one wave to the crest of the next wave. λ = nm,um, cm
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2- the waves are also characterized by Frequency (v)= number of complete cycles per second (cps), also called Hertz (Hz).
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Wavelength and frequency are inversely proportional.
• In IR ,frequency is expressed as wavenumbers
Wavenumbers have units of reciprocal cm (cm-1)
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The relationship between wavelength (or frequency) and energy (E) is well defined
1- Wavelength and frequency are inversely proportional .2- The higher the frequency, the greater the energy of the wave.3- The shorter the wavelength, the greater the energy of the wave.
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UV VIS IR RADIO wavesIncreasing wavelength- Decreasing frequency Decreasing Energy
Absorption of UV Result of promotion of electron to a higher energy level
Absorption of IR Result in increase of vibration of bonds
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Features of a spectrumAn infrared spectrum of a compound is a plot of percent transmission (%T) versus either wavelength of frequency changing.
%T = (Intensity/original intensity) x 100
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Absorption of Infrared Radiation
Cause increase the vibration of bonded atomsDifferent type of bonds ( C-H, C-C, C-O, C=O, O-H)
absorb IR at different λ.Type of vibration absorb at different λ.
R
O
H
at 3330 cm-1
Types of vibrations
1- stretching
2- bending
R
O
H
1250 cm-1
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The relative amount of absorbed energy depends on the change of bond moment
1) Non-polar bonds weak absorption
2) Polar bonds strong absorption
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The infrared spectrum
The instrument used to measure absorption of infrared radiation
infrared spectrophotometer
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Interpretation of IR spectrum
Correlation Chart
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A. c-c Bonds
• C-C single bond weak absorption (not useful)
• C=C (sp2) 1600- 1700 cm-1
• C=C( aryl, sp2) 1450-1600cm-1
C C 2100-2250 cm-1(sp)
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C-H Bonds
• (sp3) C-H 2800-3000cm-1
• (sp2) C-H (=C-H) 3000-3300 cm-1
(sp) C H ( C H ) 3300 cm-1
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Aromatic Compounds
– Aromatic Compounds The C-C bond stretching gives a set of characteristic sharp peaks between 1450-1600 cm -1
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Haloalkanes: C-X 500-1430cm-1
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Ether:C-O 1050-1260 cm-1 (strong)
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Alcohol:O-H 3000-3600cm-1
(strong) + C-O
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Hydrogen bonding O-H broadNo H-bonding O-H sharp
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Amines: RNH23000-3600cm-1
(medium or weak double peaks) + C-N (900-1300 CM-1)
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Amines: R2NH3000-3600cm-1
(medium or weak one peak) + C-N (900-1300cm-1)
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Amines: R3Nno N-H peak only C-N at 900-1300cm-1
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Carbonyl Functional Groups
Generally the carbonyl group gives a strong peak which occurs at 1630-1780 cm-1
The exact location depends on the actual functional group present
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Ketones: C=O 1680-1750 cm-1 (strong)
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Aldehydes: Carbonyl (C=O) 1720-1740cm-1
Also must show aldehyde C-H bend
Two peaks 1) 2820-2900cm-1
2 (2700-2780 cm-1
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Carboxylic acid: carbonyl (C=O)1700-1725 cm-1 (strong)
• Also must show O-H stretching very broad from 3330- 2500 cm-1
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Esters: C=O 1735-1760 cm-1
• Also shows C-O 1100-1300 cm-1
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Conclusion
• IR is used for functional groups identification.
• Not all the peaks can be analyzed.
• Example
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