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ConceptsandTheoriesVibrational frequency

FundamentalconditionforIRabsorption

InterpretingIRSpectraVibrational ModesofImportantFunctionalGroups

1

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NMR

FarIR~400 10cm1

MidIR~4000 400cm1

NearIR~14000 4000cm1

UVVis

Waveleng

th(cm)

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AbsorptionofIRRadiation

Theenergyoflightabsorbedbyasamplecorrespondstotheenergydifferencebetweendiscretevibrational orrotationallevels.

VIBRATIONAL LEVELS

electronic excited state

electronic ground stateVIBRATIONAL LEVELS

E1

E0

v1

v2

v3

v1

v2

v3

E

2 1E E E h h: Plancksconstant

: thefrequencyoflight

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Infraredspectrosopy inorganicchemistry:anempiricaltechniqueusedincomparingabsorptionsofanunknowncompoundwiththoseknownand

identifyingfunctionalgroups byanalogy.

OR OH

NR C

OO

R RR NH2

CN

R Cl

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Vibrational frequencyiscorrelatedwithbondstrengthandatomicmasses(HookesLaw):

Factorswhichaffectvibrational frequency:

Thefrequencyincreaseswithincreasingbondstrength.

stretching > bending ; CC C=C CC ?

Thefrequencydecreaseswithincreasingmassofthesubstituent.

1

2

k

: frequencyofvibration

k: theforceconstantofthebond: thereducedmassofthesystem, =(m1m2)/(m1 +m2)

OD OH ?

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Commonvibrational modes:symmetricalandantisymmetricalstretching,scissoring,rocking,waggingandtwisting.

Arrows: directionofmotionofatomsintheplaneoftheCH2 group.

+, signs:motionaboveandbelowtheplane,respectively.

TheVibrationsofaCH2 Group

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+ +

(CH2(as) (CH2(s) (CH2) (CH2)

2926cm1 2853cm1 1465cm1 13501150cm1

+ (CH2outofplane)

bendortwist

13501150cm1

(CH2) (inplane)

bendorrocking

13501150cm1

: stretching; : inplanedeformation; : outofplanedeformation.

s: symmetric; as: antisymmetric .

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AnIRactive vibrational modemustbeassociatedwithachangeinthedipolemoment.

VibrationsthatarenotIRactiveareRamanactiveandcanbe

studiedbyRamanspectroscopy.

SymmetricStretching

IRinactive

C OO

Antisymmetric Stretching

IRactive

C OO

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Characteristicabsorptionregionsforcommonlyobservedbondstretching( )

andbending( )vibrations.

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(*Bandmaybeabsentowingtosymmetry)

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Payspecificattentiontohighlightedvibrational modes

13

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(CH)

CH3 (as)2962

CH3(s)2872CH2(as)2924

CH2(s)2853

(CH)Bend

CH2 (s)1467

CH3(as)1450CH3(s)1378

(CH)Rock

CH2 721

dodecane

(CH)twist

CH2 13501150(weak)

(CC)1200800 (weak)

(CC)bendbelow500

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C=CStretchinsimplealkenes16701640

C=Cinconjugatedalkenesnear1650 and1600 (whylower?).Thisbandsplitsintotwo(s andas)innonsymmetricconjugateddienes (polyenes)suchasisopreneshownbelow.

C= C

1640 (s), w

C= C

1598(as), s( C=C-H)

(C-H)

(aliphatic C-H)

( C=C-H)

(isoprene)

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( C C)2119

CC-H)3310

(C-H) C- H bendovertone 1250

C-H)630

(1hexyne)

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Prominent(strong)absorptions:

CHstretch 3100 3000cm1

inplaneCHbend1300 1000cm1

outofplaneCHbend900 675cm1 (characteristicforsubstitutionpattern)

C=Cringstretches1600 1585and1500 1400cm1

C=Cbend450 400cm1

OvertoneandCombinationbands 2000 1650cm1 (characteristicfor

substitutionpattern)

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Determinationofsubstitutionpatternsin6memberedaromaticring:positionand

shapeofbandsrelatedtothenumberofadjacentHatoms( incm1).(Nottobeused

forringsystemswithstronglyconjugatedsubstituents suchasC=O,NO2,CC.)

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combinationbands

(overtones)

C= Car

( Car - H)

C-Har

(outofplanebending,

monosubstituted)

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IRofcyclohexylcarbinol O Hpeaks

OH(noHbonding)

orfreeOH36503584cm1

(sharpbandsingasphase)

IntermolecularHbonding

insolutionincreaseswithconcentrationandresultsina

broadabsorptionat

35503200cm1

OH

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(O -H ) (H-bonded)3331

(O-H) (overlaping withC-H in-plane bend) 1209

(C-O)

1023

benzylalcohol

OH

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(C-O)1224

(O -H )

(H-bonded)

3331

OH

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Wavenumber (cm1)

Transmission

CHaromaticC=Caromatic

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AliphaticEthers(ROR)

(COC),as,11501085cm1, (strong, branchingattheCatomnextto

Ousuallyleadstoasplitting).

(COC),s,10751000cm1, (usually weak).

Arylalkyl Ethers(ArOR)

(COC),as,12751200cm1,(strong).

(COC),s,at10751020cm1,(weaktomedium).

Epoxides

RingstretchingmodesinvolvingtheCObondarenear1250 and950810cm1.

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O CH3

COCstretch(as)

1247cm1

COCstretch(s)

1247cm1

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Ketones,aldehydes,carboxylicacids,carboxylicesters,lactones,acidhalides,anhydrides,amides,lactams allshowa (C=O)absorption(s)

between18701540cm1

.

R

C

O

R R

C

O

H R

C

O

OH R

C

O

OR

C

O

O

R

C

O

Cl R

C

O

O R

C

O

R

C

O

NH2

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(C=O)ofaliphaticketones: ~1715cm1.Conjugation toaromaticorunsaturatedgroupsshiftsthisbandtolowerenergiesby15to40 cm1.

Thereplacementofanalkylgroupbyaheteroatom(G)shiftsthe (C=O).Thedirectionoftheshiftdependsoninductiveandresonanceeffects.

Inductiveeffectsincrease (C=O)while resonanceeffectsdecreaseit.

G C=O

F 1869

Cl 18151785

Br 1812

OH(monomer) 1760OR 17501735

NH2 16951650

SR 17201690

G

C O

R

+G

C O-

R

Inductive

Resonance

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( C= O) 1717 (C-CO-C) 1171

( C= O) overtone

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Aldehydes haveslightlyhigherfrequencythanmethylketones.

( C= O)1724

(C (O) -H )

2825, 2720overtone

O

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overtoneband

( C= O)

(C (O) -H )

O

H

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Broad (OH) absorptionduetoHbonding33002500cm1.

(C=O) near1760cm1 (aliphaticmonomer)andreducedto1700cm1

(aliphaticdimers); Intramolecular HbondingisusuallystrongerandtheC=Ostretchcanbereducedto1680cm1.

O

OH

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(C=O) 1774

overtone

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( C= O)1765

(O -Ar ) , as

1193

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C

O

O C

O

O

C

O

O C

O

O

C

O

O

1674cm1

1699cm1

1760cm1

1720cm1

1800cm1 1750cm1

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( N - H )3352 (as)3198 (s)

( C= O)

1679

Outofplanebend

( N - H )700 - 600(C-N)1432

( N - H )

1617

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Nmethylacetamide (neatliquid,KBr plates)

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octylamine

( N - H )

(as) 3372,(ss) 3290

( N - H )1617

N-H wag900-700

(C-N)1073

Aliphatic C-H bands

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( C N)2226

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( S-H) , w eak2565

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( S= O)1355, 1177

( S- C- O )

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1nitromethane

(NO) as1660 to 1530

(NO) sy1390 to 1260

(C-NO) sy850

CH3NO2

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nitrobenzene

(NO) as1660 to 1530

(NO) sy

1390 to 1260

(C-NO) sy850

NO2

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Thefollowingarespectraofbenzylalcoholand1octenerecordedinchloroform.Identifyeachspectrum.

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Thefourfollowingspectrawererecordedonsolutionsofanacyl chloride,analdehyde,acarboxylicacid,andanamide.Whichcorrespondsto

which?

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Decidewhichofthefollowingspectracorrespondstobutyrolactone,cyclohexanone,ethylacetate,andmethylphenylketone.

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AcompoundwithaformulaofC3H3Clhasthe followingIRspectrum.Whatisthestructureofthiscompound?

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AcompoundwithaformulaofC3H5O2Clhasthe followingIRspectrum.Whatisthestructureofthiscompound?

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AcompoundwithaformulaofC10H12 hasthe followingIRspectrum.Whatisthestructureofthiscompound? (Hint:Itcontainstwosixmembered

rings)

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http://riodb01.ibase.aist.go.jp/sdbs/cgibin/cre_index.cgi?lang=eng

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http://riodb01.ibase.aist.go.jp/sdbs/cgibin/cre_index.cgi?lang=eng

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http://www.sigmaaldrich.com/sigmaaldrich/home.html

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http://www.sigmaaldrich.com/sigmaaldrich/home.html

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http://www.sigmaaldrich.com/sigmaaldrich/home.html