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Chapter12,MSandIR1

Chapter12InfraredSpectrophotometry

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Chapter12,MSandIR2

TheIRExperimentStretchingVibrationsAtompositionswithrespecttoeachotherarenotfixed!!Therearerelativemovementstoeachother,whichcanbedescribedbyaspring-model.Consideringtwoatomsresultsinthefollowingmodel

Energy-uptakecanbeaccomplishedbyelectromagnetradiation

StretchingandBendingVibrationsofthreeatomsa)stretching:

Asymmetrical

stretching

as

Symmetrical

stretching

s

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Chapter12,MSandIR3

In-planebending

(scissoring)

s

Out-of-plane

bending(wagging)

Out-of-plane

bending(twisting)

In-planebending

(rocking)

StretchingFrequenciesasafunctionofAtomicWeight:

C-H 3200-2800cm-1

C-O 1200-1050cm-1

C-Cl 800-650cm-1

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Chapter12,MSandIR4

StretchingFrequenciesasafunctionofBondStrengths

Bond Frequency(cm-1)

CH2 H

2960(as),2870(s)

C C

H

3020

C C H 3300

C C

1675-1650

C C

2140-2100

CH2 O 1140-1070

C O

1715

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Chapter12,MSandIR5

SymmetryisanimportantfactorofIR-spectrasinceitisassociatedwithanoscillatingelectricdipole.RingstrainTheeffectofringstraincanbeeasilyobservedinIR:

OO

O

Carbonyl-frequency:1714cm-1 1746cm-1 1783cm-1

C=Cstretching:

1649cm-1 1656cm-1 1677cm-1

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Chapter12,MSandIR6

Stereoisomers:trans-substitutedalkenes:strongbandat~970cm-1

cis-substitutedalkenes:mediumbandsat730cm1and650cm-1

ConformationalIsomers:

H

Cl

O

Cl

H

O

carbonyl:1745cm-1 1725cm-1

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Chapter12,MSandIR7

IRSpectraofAlcoholsCH3CH2OH

CH3 C

CH3

CH3

CH2 CH

CH3

CH2OH

A:O-HB:C-HC:CH2-vibration

D:C-Ostretch

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Chapter12,MSandIR8

IRSpectraofKetones

CH3 C

O

CH3

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Chapter12,MSandIR9

CH3 C

O

CH2CH2CH3

A,B,C:C-HD:C=OE,F,G:CH2-vibration

IRSpectraofAlkenes

H2C CH(CH2)7CH3

A:C-HaliphaticB:C-HolefinicC:C=CstretchD:=CHoutofplaneandCH2

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Chapter12,MSandIR10

IRSpectraofAlkynes

HC CH(CH2)3CH3

A,C,E,duetoalkyne:A:C-HalkyneC:CCstretchalkyneE:CCHbendalkyne

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Chapter12,MSandIR11

IRSpectraofAromaticCompounds

OH

A:O-HB:C-HC:OvertonesD:C=Caromaticringstretch

CH2OH

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Chapter12,MSandIR12

CH3

CH3

AromaticOvertoneandCombinationBands

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Chapter12,MSandIR13

IRSpectraofAminesN-Hstretchof1oamines:doubletat3500,3400cm-1

(verydiagnosticbutweakinintensity)N-Hstretchof2oamines:3310-3350cm-1(weak)N-Hbendof1oamines:1580-1650cm-1(mediumtostrong)N-Hwaggingof1oor2oamines:666-909cm-1(mediumtostrong,usuallybroad)CH3(CH2)6CH2NH2

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Chapter12,MSandIR14

SummaryofIRStretchingFrequenciesFrequency

(cm-1)FunctionalGroup Comments

3300 alcohol O H

alwaysbroad

amine,amide

N H maybebroad,sharp,orbroadwithspikes

alkyne C H alwayssharp

3000

alkane C H

justbelow3000cm-1

alkene C

H

justabove3000cm-1

acid C

O

O H

verybroad

2200 alkyne CC justbelow2200cm-1

nitrile NC justabove2200cm-1

1710

(verystrong)

carbonyl C O

ketones,aldehydes,acids,

estershigher(1735cm-1)amideslower;conjugation

(1650cm-1)

1600-1700

alkene C C

aromaticlower;

conjugation(1600cm-1)

imine C N

strongerthanC=C

amide C O

strongerthanC=C(seeabove)

1000-1200 etheresteralcohol

C O

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Chapter12,MSandIR15