Infra Red Spectroscopy for M Sc I (SPPU)

Infra Red Spectroscopy

1. INFRA RED SPECTROSCOPY
Shaikh SirajB.
AbedaInamdarSenior College, Pune
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2. Introduction
•Infraredspectroscopy(IRspectroscopyorvibrationalspectroscopy)
involvestheinteractionofinfraredradiationwithmatter.
•Infrared(IR)spectroscopyisoneofthemostcommonspectroscopic
techniquesusedbyorganicandinorganicchemists.Simply,itisthe
absorptionmeasurementofdifferentIRfrequenciesbyasample
positionedinthepathofanIRbeam..
•ThemaingoalofIRspectroscopicanalysisistodeterminethe
chemicalfunctionalgroupsinthesample.
•DifferentfunctionalgroupsabsorbcharacteristicfrequenciesofIR
radiation.Usingvarioussamplingaccessories,IRspectrometerscan
acceptawiderangeofsampletypessuchasgases,liquids,and
solids.
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3. Regions of EMR
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4. What happens in different regions
Region of Spectrum Wavelength Transition
X-Ray < 200 nm Bond breaking
UV/Visible 200-800 nm Electronic
Infrared 2.5-25 m Vibrational
Microwave > 25 m MW < 1 m Rotational
Radiofrequency 1-5 m NMR/ESR spin change
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5. IR radiations IR radiations
•Theinfraredportionoftheelectromagneticspectrumisusually
dividedintothreeregions;thenear-,mid-andfar-infrared,named
fortheirrelationtothevisiblespectrum.
•Thehigher-energyNearNear--IRIR,approximately14000–4000cm
−1
(0.8–
2.5μmwavelength)canexciteovertoneorharmonicvibrations.
•Themidmid--infraredinfrared,approximately4000–400cm
−1
(2.5–25μm)may
beusedtostudythefundamentalvibrationsandassociated
rotational-vibrationalstructure.
•Thefarfar--infraredinfrared,approximately400–10cm
−1
(25–1000μm),lying
adjacenttothemicrowaveregion
•Inthecontextofinfraredspectroscopy,wavelengthismeasuredin
"wavenumbers",whichhavetheunitscm
-1
5

6. 6
Energy levels
E
N
E
R
G
Y
Electronic level
Rotational level
& translational level
Vibrational level
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7. 7
Absorption -Energy levels
E
N
E
R
G
Y
E = UV/Vis photon
E = IR photon
E = wave photon
The energies associated with rotational transitions are generally much smaller than
that for vibrationaltransitions and the peaks are found below 300 cm
-1
.
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8. IR absorption Process IR absorption Process
Principle:-
•Alltheatomsinmoleculesareincontinuousvibrationwithrespectto
eachother.IfyoudirectedIRradiationtomoleculesexternally,the
frequencyofaspecificvibrationisequaltothefrequencyoftheIR
radiationdirectedonthemolecule,themoleculeabsorbstheradiation.
•Infraredspectroscopyexploitsthefactthatmoleculesabsorbfrequencies
thatarecharacteristicoftheirstructure.
•Theseabsorptionsoccuratresonantfrequencies,i.e.thefrequencyofthe
absorbedradiationmatchesthevibrationalfrequency.
•Theenergyorfrequencyabsorbedservestoincreasetheamplitudeof
thevibrationalmotionsofthebondsinthemolecules.
•Inotherword
•Infraredlightinteractsonlywiththosevibrationswhosedipolemoment
(µ)periodicallychangesduetotheoscillationoftheatom.Iftheoscillating
electromagneticfieldoftheincidentphotoncoupleswiththedipole
oscillatingatthesamefrequency,itisabsorbed.
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9. IR active / Inactive IR active / Inactive
•Notallpossiblevibrationswithinamoleculewillresultinan
absorptionbandintheinfraredregion.
••ToTobebeinfraredinfraredactiveactivethethevibrationvibrationmustmustresultresultininaachangechangeofofdipoledipole
momentmomentduringduringthethevibrationvibration..
•Thismeansthatforhomonucleardiatomicmoleculessuchas
Hydrogen(H2),Nitrogen(N2)andOxygen(O2)noinfrared
absorptionisobserved,asthesemoleculeshavezerodipole
momentandstretchingofthebondswillnotproduceone.
•ForheteronucleardiatomicmoleculessuchCarbonMonoxide(CO)
andHydrogenChloride(HCl),whichdopossessapermanentdipole
moment,infraredactivityoccursbecausestretchingofthisbond
leadstoachangeinndipolemoment
•PolarbondsareassociatedwithstrongIRabsorptionwhile
symmetricalbondsmaynotabsorbatall.
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10. •Itisimportanttorememberthatitisnotnecessaryfor
acompoundtohaveapermanentdipolemomentto
beinfraredactive.
•InthecaseofCarbonDioxide(CO
2)themoleculeis
linearandcentrosymmetricandthereforedoesnot
haveapermanentdipolemoment.
•Thismeansthatthesymmetricstretchwillnotbe
infraredactive.Howeverinthecaseoftheasymmetric
stretchadipolemomentwillbeperiodicallyproduced
anddestroyedresultinginachangingdipolemoment
andthereforeinfraredactivity.
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11. Vibration motion is QuantizedVibration motion is Quantized
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12. Molecular Molecular Vibrations Vibrations
•Foramoleculetoabsorbinfraredradiationit
mustundergoanetchangeindipolemoment
asaresultofvibrationalorrotationalmotion.
•Vibrationscanbesubdividedintotwoclasses,
dependingonwhetherthebondlengthor
angleischanging:
•stretching(symmetricandasymmetric)
•bending(scissoring,rocking,waggingand
twisting)
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13. Fundamental modes of vibration Fundamental modes of vibration
•The major types of molecular vibrations are
stretching and bending.
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14. Stretching vibrationsStretching vibrations
•Inthistypeofvibrations,thebondlengthisincreased
ordecreasedatregularintervals.Therearetwotypes
ofstretchingvibrations.Symmetricalstretchingand
asymmetricaltraining.
•A)Symmetricalstretching-Inthistypeofstretching,
bondlengthincreaseordecreasesymmetrically.
•B)Asymmetricalstretching-Inthistypeofstretching,
lengthofonebondincreasesandtheotherone
decreases.
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15. Fundamental modes of Vibrations-a) Streching
Symmetric
stretching
Antisymmetric
stretching
Direction
Radial
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16. Bending vibrations
•Inthistypeofvibrations,achangeinbondangle
occursbetweenbondswithacommonatom,or
thereisamovementofagroupofatomswith
respecttotheremainderofthemoleculewithout
movementoftheatomsinthegroupwithrespect
tooneanother.
•Thebendingvibrationsarealsocalledas
deformationvibrations.
•Deformationvibrationsareoftwotypes.
•a)In-plane Bending vibrations
•b)Out of plane Bending vibrations
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17. a)Inplanebending-Inthesetypesofvibrations,thereis
achangeinbondangle.Thistypeofbendingtakes
placewithinthesameplane.Inplanebendingareof
twotypes.
•i.Scissoringinwhichbondangledecreases
•ii.Rockinginwhichthebondangleismaintainedbut
bothbondsmoveswithinthesameplane.
b)Outofplanebending-Thistypeofbendingtakesplane
outsideoftheplanofmolecule.
•i.Wagginginwhichbothatomsmovetoonesideof
theplane
ii.Twistinginwhichoneatomisabovetheplaneandthe
otherisbelowtheplane.
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18. Scissoring
bending
Rocking
bending
Wagging
Twisting
Fundamental modes of Vibrations-b) bending
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19. CO
2
Symmetric stretchingAntisymmetricstretching
Scissoring bending
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20. CH
4
Symmetric Stretch
Symmetric Bend
Asymmetric Bend
Asymmetric Stretch
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21. Water
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22. Degrees of freedom
•Simplediatomicmoleculeshaveonlyonebond,whichmaystretch.
•Inpolyatomicmolecules,eachatomhavingathreedegreesoffreedominthreedirectionswhich
areperpendiculartoeachother.Thus,amoleculeofnatomshas3ndegreesoffreedom.
•Foralinearmolecule,twodegreesoffreedomdescriberotationandthreedegreesdescribe
translation,sotheremaining3n-5arenumberoffundamentalvibrations.Whileforanon-linear
molecule,threedegreesoffreedomdescriberotationandthreedegreesdescribetranslation,so
theremaining3n-6arenumberoffundamentalvibrations.
•Eachatomhasthreedegreesoffreedom,correspondingtomotionsalonganyofthethree
Cartesiancoordinateaxes(x,y,z).
•Apolyatomicmoleculeofnatomshas3ntotaldegreesoffreedom.However,3degreesoffreedom
arerequiredtodescribetranslation,themotionoftheentiremoleculethroughspace.Additionally,
3degreesoffreedomcorrespondtorotationoftheentiremolecule.
•Therefore,theremaining3n–6degreesoffreedomaretrue,fundamentalvibrationsfornonlinear
molecules.Linearmoleculespossess3n–5fundamentalvibrationalmodesbecauseonly2degrees
offreedomaresufficienttodescriberotation.Amongthe3n–6or3n–5fundamentalvibrations
(alsoknownasnormalmodesofvibration),thosethatproduceanetchangeinthedipolemoment
mayresultinanIRactivityandthosethatgivepolarizabilitychangesmaygiverisetoRaman
activity.
•Naturally,somevibrationscanbebothIR-andRaman-active.
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23. 23
Molecules which produce a change in dipole moment -IR active
Nonlinear molecules 3N-6 degrees of freedom
Linear molecules 3N-5 degrees of freedom
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24. Instrumentation
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25. 25SIRaJ/MScI/IR

26. What is FTIR
•Fourier-transforminfraredspectroscopyisavibrational
spectroscopictechnique,meaningittakesadvantageof
asymmetricmolecularstretching,vibration,androtationof
chemicalbondsastheyareexposedtodesignated
wavelengthsoflight.
•Fourier transform is to transform the signal from the time
domain to its representation in the frequency domain
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27. Presentation of Spectra
•IRabsorptioninformationisgenerally
presentedintheformofaspectrumwith
wavelengthorwavenumberasthex-axisand
absorptionintensityorpercenttransmittance
asthey-axis
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28. Agraphisproduced showinghowthepercentage
transmittancevarieswiththefrequencyoftheinfra-red
radiation in cm-1.
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29. n= wavenumbers (cm
-1
)
n
=`
1
l(cm)
l= wavelength (cm)
THE UNIT USED ON AN IR SPECTRUM ISTHE UNIT USED ON AN IR SPECTRUM IS
WAVENUMBERS ( WAVENUMBERS ( n n ))
Wavenumbersare directly proportional to frequency
29
UNIT ( UNIT ( n n ) = cm) = cm--
11
IRabsorptionpositionsaregenerallypresentedaseitherwavenumbers((nn))or
wavelengths(l).
Wavenumberdefinesthenumberofwavesperunitlength.
Thus,wavenumbersaredirectlyproportionaltofrequency,aswellastheenergyof
theIRabsorption.
Thewavenumberunit(cm
–1
,reciprocalcentimeter)ismorecommonlyusedin
modernIRinstrumentsthatarelinearinthecm
–1
scale
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30. 30
Percent transmission spectrum
%T
0
100
Wavelength ( or frequency )
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31. 31
A
Wavelength ( or frequency )
Absorbance Spectrum
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32. 32
%T
0
100
Wavelength ( or frequency )
% A
Absorbance SpectrumPercent transmission
spectrum
0
100
Wavelength ( or frequency )
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33. Band or peak Intensities
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34. •Theintensityofanabsorptionbanddepends
onthesizeofthechangeindipolemoment
associatedwiththevibration:Thegreaterthe
changeindipolemoment,themoreintense
theabsorption
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35. Bond properties & Absorption peaks
Bond properties Vs Frequency
•Bondpropertiessuchasbondlength,force
constantofbondandmassesofbonded
atomsaffecttheinfraredIRabsorption
frequency
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36. The natural frequency of vibration of a
bond is given by equation-(Hooke’s Law)
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37. 37
Hooke’s Law
k = force constant dyne/cm
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38. =
1
2pc
n
K

larger K,
higher frequency
larger atom masses,
lower frequency
constants
n 
From above equation wavenumberi.e. frequencry
Force constant (K)
n 
Reduced masses of atom()
As force constant increases, frequency also increases.
Stronger the bond or shorter the bond length then higher the frequency to vibrate the bond
As masses of atoms increases, frequency also decreases.
Smaller atoms required higher frequency to vibrate the bond, vice versa
1
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39. 2150 1650 1200
C=C > C=C > C-C=
increasing K
K = force constant (in dynes / cm)
•for single bond: K = 5 x 10
5
dynes/cm
•for double bond: K = 10 x 10
5
dynes/cm
•for triple bond: K = 15 x 10
5
dynes/cm
C-H > C-C > C-O > C-Cl > C-Br
3000 1200 1100 750 650
increasing 
39
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40. 40
1.Forastrongerbond(largerkvalue),nincreases
Increasingbondstrength
CCbonds CHbonds
C-C (1200cm
-1
), C-C-H(2900cm
-1
)
C=C (1650cm
-1
) C=C-H(3100cm
-1
)
CC (2150cm
-1
) CC-H(3300cm
-1
)
2.Forheavieratomsattached(largermvalue),ndecreases
Increasingreducedmasscompare
C-H (3000cm
-1
)
C-C (1200cm
-1
)
C-Cl (750cm
-1
)
C-Br (650cm
-1
)
C-I (about500cm
-1
)
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41. 41SIRaJ/MScI/IR

42. 42
•Functionalgroupregion4000-1500cm
-1
•Fingerprintregion 650-1500cm
-1
IR Spectrum
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43. 43SIRaJ/MScI/IR

44. Regions in IR spectrum
A) Functional Group Region
•TheFunctionalGroupRegion,4000to1300cm
–1
•Theappearanceofstrongabsorptionbandsintheregionof4000to
2500cm–1usuallycomesfromstretchingvibrationsbetween
hydrogenandsomeotheratomswithamassof19orless.
•TheO-HandN-Hstretchingfrequenciesfallinthe3700to2500
cm–1region,withvariousintensities.Hydrogenbondinghasa
significantinfluenceonthepeakshapeandintensity,generally
causingpeakbroadeningandshiftsinabsorptiontolower
frequencies.TheCHstretchingbandsoccurintheregionof3300to
2800cm–1.TheacetylenicC-Hexhibitsstrongabsorptionatabout
3300cm–1.AlkeneandaromaticC-Hstretchvibrationsabsorbat
3100to3000cm–1.Mostaliphatic(saturated)C-Hstretchingbands
occurat3000to2850cm–1,withgenerallyprominentintensities
thatareproportionaltothenumberofC-Hbonds.Aldehydesoften
showtwosharpaldehydicC-Hstretchingabsorptionbandsat2900
to2700cm–1.
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45. •The1950to1450cm–1regionexhibitsIRabsorptionfromawidevarietyof
double-bondedfunctionalgroups..
•AlmostallthecarbonylC=Ostretchingbandsarestrongandoccurat1870to1550
cm–1.AcidchloridesandacidanhydridesgiverisetoIRbandsat1850to1750
cm–1.Whereasketones,aldehydes,carboxylicacids,amides,andestersgenerally
showIRabsorptionat1750to1650cm–1,carboxylateionsusuallydisplay
stretchingbandsat1610to1550and1420to1300cm-1.
•Conjugation,ringsize,hydrogenbonding,andstericandelectroniceffectsoften
resultinsignificantshiftsinabsorptionfrequencies.
•NonconjugatedaliphaticC=CandC=Nhaveabsorptionbandsat1690to1620cm–
1,withvariableintensities.
•Aromaticcompoundscontaindelocalizedpelectronsfromtheresonance-
stabilizeddoublebonds,showingskeletalvibrations(includingC-Cstretchings
withinthering)inthe1650to1400cm–1regionandweakcombinationand
overtonebandsinthe2000to1650cm–1region.
•Valuableinformationaboutthesubstitutionpatternonanaromaticringcanbe
obtainedbycarefulexaminationofabsorptionbandsinthesetworegions.
MoleculescontainingNO2groups,suchasnitrocompounds,nitrates,and
nitramines,commonlyexhibitasymmetricandsymmetricstretchingvibrationsof
theNO2groupat1660to1500and1390to1260cm–1regions.
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46. B) Fingerprint Region
•TheFingerprintRegion,1300to910cm
–1
•Absorptionsinthisregionincludethecontributionsfrom
complexinteractingvibrations,givingrisetothegenerally
uniquefingerprintforeachcompound.
•AgoodmatchbetweentheIRspectraoftwocompoundsin
allfrequencyranges,particularlyinthefingerprintregion,
stronglyindicatesthattheyhavethesamemolecular
structures.
•DetailedinterpretationofIRbandsinthisregionisdifficult.
However,someassignmentsofbandsinthefingerprint
regiontoafewimportantvibrationalfrequenciesof
functionalgroupscanbedonewhenIRabsorptionsinother
regionsarecorrelatedtogether.
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47. b) FINGERPRINT REGION
AlthoughtheentireIRspectrumcanbeusedasafingerprintforthepurposesofcomparing
molecules,the600-1400cm
-1
rangeiscalledthefingerprintregion.Thisisnormallya
complexareashowingmanybands,frequentlyoverlappingeachother.Thiscomplexity
limitsitsusetothatofafingerprint,andshouldbeignoredbybeginnerswhenanalyzing
thespectrum.Asachemist,youshouldfocusyouranalysisontherestofthespectrum,
thatistheregiontotheleftof1400cm
-1
.
Fingerprint region: complex and difficult to
interpret reliably.
Focus your analysis on this region. This is where most stretching
frequencies appear.
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48. 48
1.Overtones
2.Combination bands
3.Coupling bands
4.Fermi Resonance
Spurious bands appearing in the
IR spectrum
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49. Overtones
•Invibrationalspectroscopy,anovertonebandisthespectralbandthat
occursinavibrationalspectrumofamoleculewhenthemoleculemakesa
transitionfromthegroundstate(v=0)tothesecondexcitedstate(v=2),
•Thetotalnumberofobservedabsorptionbandsisgenerallydifferentfrom
thetotalnumberoffundamentalvibrations.
•Excitationfromgroundstatetohigherenergystates,whichcorresponds
integralmultiplesofthefrequencyofthefundamental(v).
•overtonesoccursatintegralmultiplesofthefundamentalabsorption
frequencies),Weakovertonebandscouldoccurat2v,3v,….
•Overtones occur at twice the frequency (2n) of a
fundamental vibration
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50. Overtones occur at twice the frequency (2n) of a fundamental
vibration
22--ButanoneButanone
CH
3
CCH
2
CH
3
O
C=O
C-H
CH bend
3438
overtone of strong C=O peak
1719 x 2 = 3438
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51. 51SIRaJ/MScI/IR

52. Combination bands
•Combinationbandsareofweakintensity
andoccurwhentwofundamentalbands
coupletogivebandsat(n
1+n
2)or(n
1-n
2)
•Twovibrationalfrequencies(v
1andv
2)inamolecule
coupletogiverisetoanewinfraredactivefrequency.
Thisbandisthesumofthetwointeractingbands
(v
combination=v
1+v
2).
•combinationbandsareobservedwhenmorethantwo
ormorefundamentalvibrationsareexcited
simultaneously.
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53. 53
Combination bands are of weak intensity and occur when two
fundamental bands couple to give bands at (n
1+ n
2)or (n
1-n
2)
ChlorobenzeneChlorobenzene
Cl
C-Cl
benzene
C=C
benzene ring
combination
bands
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54. 54
Combination bands
IR Spectrum of Butyramide
NH
2
O
Hence, combination bands are not of diagnostic
value in IR interpretation.
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55. Couplingbandsoccurswhentwosimilarfunctionalgroupsareinclose
proximityandtheyhavestrongfundamentalabsorptions.
Coupling bands
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56. 56
Coupling bands occurs when two similar functional groups
are in close proximity and they have strong fundamental
absorptions.
IR Spectrum of Benzoic anhydride
O
O O
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57. Fermi Resonance
•Whenafundamentalvibrationcoupleswithan
overtoneorcombinationband,thecoupledvibrationis
calledFermiresonance.Fermiresonanceisoften
observedincarbonylcompounds.
•Fermiresonanceresultsinthesplittingoftwo
vibrationalbandsthathavenearlythesameenergy
andsymmetryinbothIRandRamanspectroscopies.
•Thetwobandsareusuallyafundamentalvibrationand
eitheranovertoneorcombinationband
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58. 58
Fermi Resonance occurs when a fundamental vibration
couples with a combination band or an overtone.
IR Spectrum of Benzoyl chloride
O
Cl
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59. Important functional group & IR
frequency
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60. 60SIRaJ/MScI/IR

61. 61SIRaJ/MScI/IR

62. Alkane
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63. 3000 divides
UNSATURATED
SATURATED
•C-H spstretch ~ 3300 cm
-1
•C-H sp
2
stretch > 3000 cm
-1
•C-H sp
3
stretch < 3000 cm
-1
The C-H stretching region
BASE VALUE = 3000 cm
-1
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64. 64
HexaneHexane
CH
3
CH
2
CH
2
CH
2
CH
2
CH
3
CH
stretching
vibrations
ALKANE
includes
CH
3
sym and asym
CH
2
sym and asym
CH bending vibrations
discussed shortly
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65. 65
Hexane
CH
3
CH
2
CH
2
CH
2
CH
2
CH
3
CH
stretch
CH
2
bend
CH
3
bend
CH
2
rocking
> 4C
ALKANE
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66. Alkene C=C
•Normal charecteristicC=C frequency occurs at 1620-
1640 cm
-1
•Aromatic C=C frequency occurs at range of 1400-1500
cm
-1
•Exocyclicdouble bond variable absorption frequency
depending on ring.
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67. 67
11--HexeneHexene
CH
2
CHCH
2
CH
2
CH
2
CH
3
=CH
CH
C=C
CH
2
CH
3
bend
CH oops
ALKENE
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68. 68
TolueneToluene
CH
3C=C
benzene
CH
3
Ar-H oops
Ar-H
AROMATIC
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69. ExoC=C bonds
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70. Endo double bond
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71. Alkynes C≡C
•C≡C alkynes at 2100-2150 cm
-1
•C≡C-H terminal C-H at 3300
cm-1
•If any compundhave this two peak in IR, the
compundhaving terminal alkynegroup
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72. 72
11--HexyneHexyne
CH CCH
2
CH
2
CH
2
CH
3
C=C=
=C-H=
C-H
CH
2, CH
3
ALKYNE
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73. Carbonyl –C=O
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74. 74
CR
O
H
CR
O
O
CR
O
CR
O
Cl
CR
O
OR'
CR
O
R
CR
O
NH
2
CR
O
OH
169017101715172517351800
1810 and 1760
BASE
VALUE
acid
chloride ester aldehyde
carboxylic
acid amideketone
anhydride
( two peaks )
EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY
C=O IS SENSITIVE TO ITS ENVIRONMENTC=O IS SENSITIVE TO ITS ENVIRONMENT
THESE VALUES ARE
WORTH LEARNING
all are +/-10 cm
-1
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75. Every type of carbonyl compound has other places you can look to confirm
your conclusion based on
frequency alone.
CONFIRMATION OF FUNCTIONAL GROUPCONFIRMATION OF FUNCTIONAL GROUP
RC
O
H
C=O at 1725 cm
-1
also look for aldehyde CH
2850 and 2750 cm-1
RC
O
OH
C=O at 1710 cm
-1
also look for OH
(H-bonded) and
C-O ~1200 cm-1
RC
O
NH
H
C=O at 1690 cm
-1
also look for two
NH peaks at
3400 cm-1
RC
O
OR'
C=O at 1735 cm
-1
also look for two
C-O at 1200 and
1000 cm-1
Ketoneshave C=O at 1715 cm
-1
and no NH, OH, C-O or -CHO
Anhydrides have twoC=O peaks near 1800 cm
-1
and two C-O
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76. Aldehyde
•C-H aldehyde, two peaks (both weak)
~ 2850 and 2750 cm
-1
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77. Ketone
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78. 78SIRaJ/MScI/IR

79. Ring Strained & IR frequency
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80. Ester
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81. Amide
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82. 82SIRaJ/MScI/IR

83. Factor affecting IR frequency
83SIRaJ/MScI/IR

84. +I & -I
84SIRaJ/MScI/IR

85. Ring size
•Underidealconditionsthecarbonatomofacarbonylgroupis
essentiallysp2hybridized,whichimpliesthatthebondangleswill
be120°andtheC-Osigmabondhas33%s-character.
•Ifthisgroupisincorporatedinasmallring,theC-CO-Cbondangleis
reducedto108°(5-memberedring),90°(4-memberedring)or60°
(3-memberedring).
•Whenthishappens,theC-Cbondsoftheringassumegreaterp-
characterandtheC-Osigmabondhascorrespondinglygreaters-
character.Thedoublebondofthecarbonylgroupistherefore
shorterandstronger,andexhibitsalargerstretchingfrequency.
85SIRaJ/MScI/IR

86. 86SIRaJ/MScI/IR

87. Conjugation or +R effect
87SIRaJ/MScI/IR

88. •Conjugationextendsthedipolarcharacterofthecarbonyl
grouptothedoublebond(oraromaticring)sothatthe
beta-carbonatomsharesthepositivecharacterofthe
carbonylcarbon.Asillustratedbythefollowingresonance
equation,thisthecarbonyldoublebondhasslightlymore
singlebondcharacterthandoesanunconjugatedfunction.
Thebondenergy(andforceconstant-f)oftheconjugated
C=Ogroupiscorrespondinglyreduced,andthisresultsina
lowerstretchingfrequency.
88SIRaJ/MScI/IR

89. 89SIRaJ/MScI/IR

90. •Electronwithdrawinggroupshaveanopposite
influence,andincreasethestretchingfrequencyofthe
carbonylgroup.Trichloroacetaldehyde(leftbelow)
providesagoodexample.
•Electrondonatingsubstituentsonthecarbonylgroup
stabilizetheionicresonancecontributor,andincrease
thesinglebondcharacteroftheC=Obond.The
stretchingfrequencyisthereforedecreased,asnoted
intherighthandexamplebelow.
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91. Ester
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92. 92SIRaJ/MScI/IR

93. Ester with cross conjugations
93SIRaJ/MScI/IR

94. Acid chlorides
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95. Anhydrides
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96. EAA
96SIRaJ/MScI/IR

97. 97SIRaJ/MScI/IR

98. 1715174517801815
1690
RING STRAIN CONJUGATION
CONJUGATION AND RING SIZE EFFECTSCONJUGATION AND RING SIZE EFFECTS
1705
CR
O
R
CR
O
CHCH2
CR
O
OOOO O
normal
aliphatic
ketones
98SIRaJ/MScI/IR

99. 99
O O
N
H
O
O O
O O
1735
1660
1735
1760
O O
N
H
O
O O
O O
1770
1705
1770
1800
O
O
NH
O
1820
1745
SIRaJ/MScI/IR

100. 100SIRaJ/MScI/IR

101. 101SIRaJ/MScI/IR

102. Field effect
102SIRaJ/MScI/IR

103. --halo halo ketoketorulerule
•See in Field effect….
103SIRaJ/MScI/IR

104. Amines
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105. Nitro –NO
2
•TheN–Ostretchingvibrationsinnitroalkanesoccur
near1550cm
-1
(asymmetrical)and
• 1350cm
-1
(symmetrical),
•thebandat1550cm
-1
beingthestrongerofthetwo.
•ifthenitrogroupisattachedtoanaromaticring,
theN–Ostretchingbandsshifttodowntoslightly
lowerwavenumbers
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106. -OH
106SIRaJ/MScI/IR

107. •TheO–Hbandsoccurathigherfrequency,sometimesasasharp
absorptionatabout3600cm
–1
.Moreoften,youwillseeabroad
absorptionatanywherefrom3500to2900cm
–1
.Thisisbecause
OHgroupsformstronghydrogenbondsthatvaryinlengthand
strength.Thesharpabsorptionat3600cm–1isthenon-hydrogen-
bondedOHandthelowertheabsorptionthestrongertheHbond.
•Alcoholsformhydrogenbondsbetweenthehydroxyloxygenofone
moleculeandthehydroxylhydrogenofanother.Thesebondsare
variableinlength(thoughtheyareusuallyratherlongerthan
normalcovalentO–Hbonds)andtheyslightlyweakenthetrue
covalentO–Hbondsbyvaryingamounts.Whenabondvariesin
lengthandstrengthitwillhavearangeofstretchingfrequencies
distributedaboutameanvalue.Alcoholstypicallygivearounded
absorptionatabout3300cm–1
107SIRaJ/MScI/IR

108. Hydrogen bonding
108SIRaJ/MScI/IR

109. Carboxylic -COOH
•Carboxylicacids(RCO2H)formhydrogen-bonded
dimerswithtwostrongHbondsbetweenthe
carbonyloxygenatomofonemoleculeandthe
acidichydrogenoftheother.Thesealsovary
considerablyinlengthandstrengthandusually
giveverybroadV-shapedabsorbances.
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110. Free -OH and bonded -OH
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111. Effect of solvent on –OH frequency
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112. Ether REther R--OO--R or R or CC--OO
1100 to 1200 cm
-1
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113. Nitrite Nitrite --C≡C≡NN
2210 to 2250 cm
-1
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114. Aromatic & substituents
•In between following values
•1400 cm
-1
•1500 cm
-1
•1600 cm
-1
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115. Applications
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116. Questions
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117. Arrange by Increasing order of IR frequency
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118. Decreasing order of Carbonyl frequency
SIRaJ/MScI/IR 118

119. SIRaJ/MScI/IR 119

120. Progress of reaction / Monitor of reactions
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121. SIRaJ/MScI/IR 121

122. Distinguish by IR spectroscopy
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123. SIRaJ/MScI/IR 123

124. SIRaJ/MScI/IR 124

125. 125SIRaJ/MScI/IR

126. SIRaJ/MScI/IR 126

127. •predict which will have the highest carbonyl
stretching frequency and which will have the
lowest carbonyl stretching frequency. Explain
your reasoning.
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