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WWU Chemistry
INFRARED SPECTROSCOPY
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REGION PROCESS
X-rayUV/VisibleInfraredMicrowaveRadio Frequency (nmr)
Bond-breakingElectronicVibrationalRotationalNuclear andElectronic Spin
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Infrared Spectrometer
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Molecular vibrations
• IR-Tutor available in CB 280 is great!
• Basically two types
stretching vibrations
bending vibrations
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IR TUTOR
• Select Chemistry Applications icon
• Select Structure Drawing icon
• Select Spectroscopy icon
• Select IR Tutor icon
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Typical Infrared AbsorptionRegions
C-H stretching region
• Alkanes C-H sp3 stretch < 3000 cm-1
• Alkenes C-H sp2 stretch > 3000 cm-1
• Alkynes C-H sp stretch ~ 3300 cm-1
C-H bending region
• CH2 bending ~ 1460 cm-1
• CH3 bending (asym) appears near the same value
• CH3 bending (sym) ~ 1380 cm-1
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Hexane
CH3 CH2 CH2 CH2 CH2 CH3C-H stretch
CH2 bend
CH3 bend
How do you think the infrared spectrum of polyethylene would differ from the spectrum of hexane?????
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1650 - 1450 cm-1
• The C=C is found at about 1650 cm-1. Monomers would be expected to show the C=C bond, but in polymers the double bond will be absent, unless there is a benzene ring.
• The double bonds in a benzene ring are found in the range of about 1600 to 1450 cm-1
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1-Hexene
CH2 CH CH2 CH2 CH2 CH3
sp2
C-H
sp3
C-H stretch
C=C stretch
out of planebendings (oops)
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Toluene
CH3
sp2
C-Hsp3
C-H aromatic C=C
aromatic oops
O-H stretching region
• O-H 3300 cm-1 (alcohol). Common polymers with O-H: cellulose and PVA
• O-H 3300 cm-1 (acid, broad and ugly). Dicarboxylic acids are monomers. When the carboxylic acid is polymerized with an diol, you get an polyester.
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Cyclohexanol
OHO-Hstretch
bending
C-Ostretch
sp3 C-H stretch
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Butanoic Acid
CH3 CH2 CH2 C OH
O
O-H stretch C=O stretch
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2250 cm-1
• C≡N 2250 cm-1
The only common polymers with this functionalgroup: polyacrylonitrile and ABS
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Propanenitrile
CH3 CH2 C Nsp3 C-H C≡N
N-H stretching region
• 3300 - 3400 cm-1
• –NH2 Primary amines give a pair of peaks. You don’t see primary amines in polymers.
• -NH Secondary amines give a single peak
The N-H in polyamides and polyurethanes show a very prominent singlet peak!
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1-Butanamine
CH3 CH2 CH2 CH2 NH2
N-Hstretchdoublet
sp3 C-H stretch
N-Hbend CH2
CH3
bend
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3-Methylbenzenamine
NH2
CH3
N-Hstretch
sp2 & sp3
C-H
N-H bendAromatic C=C
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N-Ethylbenzenamine
NH CH2 CH3
N-Hstretch
sp2 & sp3 C-H
Aromatic C=C
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1770 - 1670 cm-1
• This is the carbonyl region!! All bands are very strong!!
• The following polymers would be expected to show prominent C=O peaks:
• Polyesters
• Acrylics
• Polyamides and polyurethanes
• Poly ether ketone (PEEK)
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C=O Stretching in monomers
and polymersFunctional Group Frequency (cm-1)
Ester Ketone Carboxylic acid Amide
1735 to 1700 1715 to 1680 1710 1690
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2-Butanone
CH3 C CH2 CH3
O
sp3 C-Hstretch
C=O stretch
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4-Methyl-2-pentanoneC-H < 3000, C=O @ 1715 cm-1
CH3 CH CH2 C CH3
OCH3
C-H stretch C=O stretch
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Ethyl Butanoate
CO
O
CH2 CH2 CH3CH2CH3
sp3 C-H
C=O stretch
C-O stretch
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Butanoic Acid
CH3 CH2 CH2 C OH
O
O-H stretch C=O stretch
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Propanamide
CNH2
O
CH2CH3N-H stretch (pair)
N-H bend
sp3 C-H
C=O and
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Conjugation of C=O with C=C
• Conjugation of a carbonyl with a C=C bond shifts values to lower frequencies
• For ketones and esters, subtract about 30 cm-1 for conjugation with C=O
• Conjugated ketone = 1690 to 1680 cm-1
• Conjugated ester = 1710 to 1700 cm-1
• C=C becomes quite strong!!
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4-Methyl-3-penten-2-one
C C
CH3
CH3
C CH3
O
H
C-H stretch
C=O stretch
C=Cstretch
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Acetophenone
C CH3
O
C-H stretch
conj C=O
aromatic C=C
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C-X stretching region
• C-Cl 785 to 540 cm-1, often hard to find amongst the fingerprint bands!!
• C-Br and C-I appear outside the useful range of infrared spectroscopy.
• C-F can be found easily, but aren’t that common - exception! polytetrafluoroethylene
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Carbon Tetrachloride
CCl Cl
Cl
Cl
no C-H!
C-Cl stretch