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INFRARED SPECTROSCOPY

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REGION PROCESSX-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 vibrationsbending 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=Oaromatic 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