intensity

1

Vibrationalspectroscopy

6.3.7AbsorbantintensityandIRspectroscopy

Variationofdipolemoments:Accordingtoquantumtheory,theabsorption

intensityisproportionaltothesquarevalueofthevariationofvibrational

mpleC＝Ohasastrongpeak,whileC=Cpeakis

weakandsometimevendisappears.

Theintensityofvibrationisrelatedtovariationofdipolemoments,

symmetryofthebondandalsotypeandconcentrationofsolvents.

weakandsometimevendisappears.

Symmetryofthebond:Forthesametypeofmolecule,thevariationofdipole

mmetricalstructure

mple,R—CH＝CH2molarabsorption

coefficientAis40,whilecis-R—CH＝CH—R′is10,trans-R—CH＝CH—

R′is2.

Typeandconcentrationofsolvents:Absorptionintensitydiffersindifferent

solventsduetothecombinationstate.

Vibrationalspectroscopy

6.3.7AbsorbantintensityandIRspectroscopy

t

r

a

n

s

mi

t

t

a

n

c

e

Fig.6-18IRspectrum.s（strong）、m（medium）、w

（weak）、vw（veryweak）

a

b

s

o

r

b

a

n

c

e

Vibrationalspectroscopy

6.3.7AbsorbantintensityandIRspectroscopy

%100%

0

×=

I

I

T

Y-coordinatecanbeprentedas

1．Transmittance

whereistheincidentintensity,IistheintensityoftheI

0

istheincidentintensity,Iistheintensityofthe

transmittedlight.

2．Absorbance

x-coordinate：wavelengthorwavenumber,wavenumber

isthereciprocalofwavelength.

ν（cm-1）=104/λ(μm)

I

I

T

A0lg

1

lg==

Vibrationalspectroscopy

6.3.8Groupfrequencies

Vibrationalfrequenciesofparticularchemicalgroupsinmolecules

tendtobehavelargelyindependentlyoftherestofthemoleculeof

orptionfrequenciesforaparticular

chemicalgroupareesntiallyconstantandarealwayscharacteristic,

thereforenamedasgroupfrequencies.

¾Mostinvestigationsrelyonamoreempiricalapproach;

¾Absorptionbandsappearinroughlythesameregionofthe

spectra;

¾Assignparticularabsorptionbandstovibrationsinthegroupsby

referencetostandardcorrelationtables.

¾Computerdataba---automaticassignment

Vibrationalspectroscopy

Typeofvibration

Characteristicfrequencies(cm-1)

O─H{stretching}

3650-3000

N─H{stretching}

3500-3300

─H

3200-2800

RegionI:StretchingofX-H,4000-2500cm-1

C{stretching}

32002800

S─H{stretching}2600-2550

≡C─H{stretching}

3100

=C─H{stretching}3040-3010

Vibrationalspectroscopy

3334cm-1–range:

3350±avery

he

peaksduetotheOHgrouparebroaddue

tohydrogenbonding.

2

Vibrationalspectroscopy

3390cm-1–NH

2

antisymmetricstretch.

Normalrange:3300±

weakeradsorptionthantheOHstretch

inhexanol.

Vibrationalspectroscopy

3290cm-1–NH

2

symmetricstretch.2°

amineshaveonlyoneNHstretch,and

3°amineshavenone.

Vibrationalspectroscopy

Spectralinterpretationalwaysstartsatthehigh

end,becautherearethebestgroupfrequencies

s

appearabove3000cm-1,thecut-offfor

rpeaksbelow3000cm-1

aresaturatedC-Hstretchingmodes.

Vibrationalspectroscopy

3050±50cm-1correspondstothearomaticor

unsaturatedC(sp2)-above

andsarenotassignedto

specificvibrationalmodes.

Vibrationalspectroscopy

3080cm-1=CH3080cm1

2

absorptionabove3000cm-1

indicatestheprenceofan

unsaturation(doubleor

triplebondoranaromatic

ring).

Vibrationalspectroscopy

C≡N{stretching}2260-2220

C≡C{stretching}2260-2100

N{stretching}23102135

RegionII:StretchingofC≡C,-C=C=C,N≡N,C≡N,N=C=O

2500-1900cm-1

N≡N{stretching}2310-2135

C=C=C{stretching}1950

N=C=O{stretching}2260-2242(verystrong)

3

Vibrationalspectroscopy

2247cm-1C≡range:

2250±10cm-1,lowered10-20cm-1

etotheC≡C

stretchin1-heptyne(3000cm-1).

Vibrationalspectroscopy

RegionIII:StretchingofC=C,-C=O,─NO

2

,S=O,1900-

1200cm-1

C=C{stretching}1680-1620

C=C{stretching}─aromatic1600,1580

1500,1450

C=O{stretching}─aromatic1850-1600

─NO

2

{stretching}1600-1500{asymmetric}

1300-1250{symmetric}

S=O{stretching}1220-1040

CO{stretching}aromatic18501600

Vibrationalspectroscopy

1742cm-1,-C=lring

esters,thisvibrationisshiftedtohigher

frequencybycouplingtothestretchof

amountofcouplingdependsontheO-

C(O)-othercarbonyl

groups,conjugationlowersthefrequency.

Vibrationalspectroscopy

1642cm-1,C=

range:164020cm-1forcisandrange:1640±20cmforcisand

vinyl,1670±10cm-1fortrans,triand

-2-hexene

(overlaymenu)hasonlyaveryweak

absorption,becauthereisvery

littledipolechangewhenaninternal

doublebondstretches(itisnearly

symmetric).

C=S{stretching}1250-1000

CH

2

{bending}1470-1400

CH

3

{bending}1380

C─C{aromatic─stretching}1600,1580,1500,1450,1000

RegionIV:<1650cm-1

C─C{alicyclic─stretching}1300-600

C─O─C{stretching}1150-1060{asymmetric}970-800{symmetric}

Si─C─Si{stretching}1110-1000{asymmetric}550-540{symmetric}

O─O{stretching}900-845

S─S{stretching}550-430

C─(CH

2

)

n

n≥4rocking

720,(n=1,775)

Vibrationalspectroscopy

Thebroadpeakatapproximately1460cm-1

1640±10cm-1,theantisymmetricbendof

theCH

3

a

degeneragebend(oneshown).

4

Vibrationalspectroscopy

At1375±10cm-1,theCH3symmetricbend(alsocalledthe

“umbrella”bend)akisveryufulbecau

ethespectrumof

tprominentdifferencebetweenthe

twospectraistheabnceofaCH3symmetricbendinthe

cyclohexanespectrum.

Vibrationalspectroscopy

Se─Se{stretching}330-290

C(aromatic)─S

{stretching}

1100-1080

C(alicyclic)─S

{stretching}

790-630

C─Cl{stretching}800-550

C─Br{stretching}700-500

C─I{stretching}660-480

C─C{blending}400-250

latticevibrations─

accordionmodles

200-20

Vibrationalspectroscopy

AssignmentofIRspectroscopy

¾Frequency;

¾Intensity;

¾Shapeofpeak.

Vibrationalspectroscopy

Factorsinfluencingthegroupfrequencies

Externalfactor:includingstateofmolecules,polarityof

mple,higherfrequencieswillbe

ore,itisimportanttonoticethe

stateandsamplepreparationmethodwhenreferringto

standardspectrumstandardspectrum.

Internalfactor:includingelectroniceffectsandhydrogen

bondingandetc.

Vibrationalspectroscopy

Factorsinfluencingthegroupfrequencies

Electroniceffectssuchasinductiveeffects,conjugativeeffect

changethedistributionofelectroncloud,resultingthe

changingofforceconstantandabsorbantfrequency.

(1)Inductiveeffects

/cm-1171518CO

ν

=

Theforceconstantincreasduetotheattractingeffectofhigher

electronegativeatomFandCl,resultinginhighergroup

frequencies.

Vibrationalspectroscopy

Factorsinfluencingthegroupfrequencies

(2)conjugativeeffect

Conjugativeeffectleadstodecreasingofforceconstantdueto

flatteningofelectronicdensity,resultinginlowergroup

frequencyfrequency.

5

Vibrationalspectroscopy

Factorsinfluencingthegroupfrequencies

(2)Hydrogenbonding

/cm-117601700CO

ν

=

Hydrogenbondingalsoleadstodecreasingofforceconstant

duetoflatteningofelectronicdensity,resultinginlower

groupfrequency.

Vibrationalspectroscopy

Coupling

¾Couplingofgroupscausshiftofspectra

¾Shiftsofspectraprovidevaluableinformationonthemolecular

itfthtilhil

Factorsinfluencingthegroupfrequencies

environmentoftheparticularchemicalgroup

¾Themagnitudeofthecouplingeffectissmallestwhenthe

atomsofneighboringgroupshavesignificantlydifferent

massorwhenthebondvibrationsposssrelativelylarge

forceconstants.

Vibrationalspectroscopy

Coupling

¾Theforceconstantsaregreaterforbondstretchingvibrations

thanforbendingdeformations;

¾Theabsorptionfrequenciesforstretchingmodesareesntially

invariantwhereasthoforbendingmodesshowmore

significantshiftsduetocoupling;

¾Formosthydrocarbons,assignmentofspecificvibration

frequenciestoparticulargroupmotionsisusuallypossible;

¾Incaoffluorinatedpolymers,thecouplingofvibrationand

bendingmotionsleadstogroupvibrationswhicharevery

nsitivetotheeffectsofsubstitution;