162 Lecture Notes Chem 51A S. King Chapter 13 Infrared Spectroscopy I. Background Nearly every portion of the electromagnetic spectrum has been used to elucidate the structures of atoms and molecules. The Electromagnetic Spectrum: A variety of techniques are available, including Ultraviolet/Visible (UV/Vis) Infrared (IR) and Nuclear Magnetic Resonance (NMR) Spectroscopy. These techniques are based on the fact that molecules have different kinds of energy levels, and therefore absorb radiation in several regions of the electromagnetic spectrum. When a molecule absorbs light of a given frequency, specific molecular effects occur, depending on the wavelength absorbed. Low energy radiowaves, for example, cause nuclear spin flip transitions, whereas more energetic UV radiation results in electrons being promoted to higher energy levels. Energy is proportional to the frequency of light absorbed: 10 !2 10 0 10 2 10 4 10 6 10 8 10 10 10 12 10 20 10 18 10 16 10 14 10 12 10 10 10 8 10 6 10 4 radiowave microwave infrared X-ray "!ray visible UV 400 nm 750 nm visible spectrum # (s !1 ) $ (nm)
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162
Lecture Notes Chem 51A
S. King
Chapter 13 Infrared Spectroscopy
I. Background Nearly every portion of the electromagnetic spectrum has been used to elucidate the structures of atoms and molecules. The Electromagnetic Spectrum: A variety of techniques are available, including Ultraviolet/Visible (UV/Vis) Infrared (IR) and Nuclear Magnetic Resonance (NMR) Spectroscopy. These techniques are based on the fact that molecules have different kinds of energy levels, and therefore absorb radiation in several regions of the electromagnetic spectrum. When a molecule absorbs light of a given frequency, specific molecular effects occur, depending on the wavelength absorbed. Low energy radiowaves, for example, cause nuclear spin flip transitions, whereas more energetic UV radiation results in electrons being promoted to higher energy levels. Energy is proportional to the frequency of light absorbed:
!
10!2 100 102 104 106 108 1010 1012
1020 1018 1016 1014 1012 1010 108 106 104
radiowavemicrowaveinfraredX-ray"!ray
visi
ble
UV
400 nm 750 nm
visible spectrum
# (s!1)
$ (nm)
163
Molecular effects associated with different regions of the EM spectrum:
II. IR Spectroscopy IR radiation causes groups of atoms to vibrate with respect to the bonds that connect them.
We will mostly be concerned with stretching vibrations because bending vibrations occur in a
region of the spectrum that is difficult to interpret.
Wavelength (!)
10"10 meter
10"8 meter
10"6 meter
10"4 meter
10"2 meter
100 meter102 meter
gamma rays
X-rays
vaccum UV
near UV
visible
infrared (IR)
microwave
radio
106 kcal
104 kcal
102 kcal
10 kcal
1 kcal
10"6 kcal10"4 kcal10"2 kcal
ionization
electronic transitions
molecular vibrations
rotational motion
nuclear spin transitions
Energy/mole Molecular effects
C
O
H
HH
rest of molecule
164
A. The frequency of the stretching vibration depends on two quantities: m1 and m2: the masses of the atoms f: the force constant, or spring constant (a measure of the stiffness of the bond)
• Heavier atoms vibrate more slowly than lighter ones. • The force constant is larger for stronger bonds (it takes more energy to
stretch a stronger bond) and smaller for weaker bonds, thus stronger bonds usually vibrate at a higher frequency than weaker bonds between similar atoms.
Examples:
O H
C H
1.
2.
3. H Csp
H Csp2
H Csp3
165
B. The intensity of the absorption is proportional to the strength of the dipole moment of the molecule. ∴ A symmetrical bond with no dipole moment will show only a weak absorption or none at all.
C. For a given type of bond, the frequency shows little change with the rest of the molecule.
∴ IR is especially useful in indicating the functional groups present in a molecule. How do you interpret an IR spectrum?
1) You will usually be given the molecular formula of the compound. Use it to evaluate the units of unsaturation (see Chapter 10.)
2) Divide the spectra into 3 regions, and look for characteristic bands using
the IR chart given: X-H region sp region double bond region
3) Practice, practice, practice!
CH3H3C
O H
CH
H
R
H
C O!+ !"
R
H
C O!+ !"
R
H
C C
R
H
C C
R
H
R
H
vs.
polar bond non-polar bond
166
Interpretation of Infrared Spectra
Aldehydes
Carboxylate
Amides
Acids
Esters
Acid Chlorides
Anhydrides
1725
1550-1610 and 1400
1650
1710
1725 - 1750
1800
1800 - 1840 (2 bands)
1350 and 1550
1600 - 1700
1600 - 1670 weak unless conjugatedDoubleBondRegion
1740
1715
1680
Conjugation:subtract 30
Ring Contraction:add 30 for each contraction
1770
1770
1740
1710andAcyclicKetones:
Carbonyl Groups:
Nitro
Imines
Alkenes
spRegion
(weak or absent if highly symmetrical)
2350
2200
2100
Carbon dioxide
Nitriles
Acetylenes
X-HRegion
secondary
primary
2700 -2800
2900 - 3000
3000 - 3200
3200 -3300
3300 - 3500
2800 - 3600 very broad
3700 - 3500 sharp or3200 - 3600 broad (H-bonded)
1 band:
2 bands:
C-H Bonds
Amines &Amides
Acids
Phenol &Alcohols RO H
R O
R N
HN
C C
C C
O
O
H
H
H
C C H
H
C C
O
HR
C N
NHH
NRH
O C O
C N
C C
NO2
O OO
O
OCl
O
O O
O
OO
OO
O
O
OR
OH
O
O
NH2
O
O
O
H
167
168
OH
OH
O
169
O
OOH
NH2
170
CN
Br
NH2
O
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