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INFRARED SPECTROSCOPY. WWU Chemistry. WWU Chemistry. Infrared Spectrometer. WWU Chemistry. Molecular vibrations. IR-Tutor available in CB 280 is great! Basically two types stretching vibrations bending vibrations. WWU Chemistry. IR TUTOR. Select Chemistry Applications icon

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molecular vibrations
Molecular vibrations
  • IR-Tutor available in CB 280 is great!
  • Basically two types

stretching vibrations

bending vibrations

WWU Chemistry

ir tutor
IR TUTOR
  • Select Chemistry Applications icon
  • Select Structure Drawing icon
  • Select Spectroscopy icon
  • Select IR Tutor icon

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c h stretching region
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
C-H bending region
  • CH2 bending ~ 1460 cm-1
  • CH3 bending (asym) appears near the same value
  • CH3 bending (sym) ~ 1380 cm-1
hexane
Hexane

CH3 bend

CH2 bend

C-H stretch

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slide10

How do you think the infrared

spectrum of polyethylene would

differ from the spectrum of

hexane?????

1650 1450 cm 1
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
1-Hexene

sp2

C-H

C=C

stretch

out of plane

bendings (oops)

sp3

C-H stretch

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toluene
Toluene

sp2

C-H

sp3

C-H

aromatic

C=C

aromatic oops

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o h stretching region
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.
cyclohexanol
Cyclohexanol

bending

O-H

stretch

C-O

stretch

sp3 C-H stretch

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butanoic acid
Butanoic Acid

O-H stretch

C=O stretch

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2250 cm 1
2250 cm-1
  • C≡N 2250 cm-1

The only common polymers with this functional

group: polyacrylonitrile and ABS

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propanenitrile
Propanenitrile

sp3 C-H

C≡N

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n h stretching region
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!

1 butanamine
1-Butanamine

N-H

bend

CH2

CH3

bend

N-H

stretch

doublet

sp3 C-H stretch

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3 methylbenzenamine
3-Methylbenzenamine

sp2 & sp3

C-H

N-H

stretch

Aromatic

C=C

N-H bend

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n ethylbenzenamine
N-Ethylbenzenamine

N-H

stretch

sp2 & sp3 C-H

Aromatic C=C

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1770 1670 cm 1
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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2 butanone
2-Butanone

sp3 C-H

stretch

C=O stretch

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4 methyl 2 pentanone c h 3000 c o @ 1715 cm 1
4-Methyl-2-pentanoneC-H < 3000, C=O @ 1715 cm-1

C-H stretch

C=O stretch

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ethyl butanoate
Ethyl Butanoate

C-O stretch

sp3 C-H

C=O stretch

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butanoic acid1
Butanoic Acid

O-H stretch

C=O stretch

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propanamide
Propanamide

sp3 C-H

C=O and

N-H stretch

(pair)

N-H bend

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conjugation of c o with c c
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
4-Methyl-3-penten-2-one

C-H stretch

C=O

stretch

C=C

stretch

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acetophenone
Acetophenone

C-H stretch

aromatic C=C

conj C=O

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c x stretching region
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
Carbon Tetrachloride

no C-H!

C-Cl stretch

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