for B.Sc. Students

1. Infra–RedSpectroscopy
Dr.SatishChandraSati
(DepartmentofChemistry)
E-mail:sati_2009@rediffmail.com
WhenlightofIRrangeintrectwithmatteritgeneratevibrationallevelinthemolecule.
AndstudyofgeneratedspectrumiscalledvibrationalspectroscopyorIRspectroscopy.
Theory–moleculehavinguneqaldistributionofmassresponsibleforthegeneration
vibrationinthemoleculeforexampleCH3OHmoleculehasthreeatom asC,H,andOif
OxygenisregaredascentrepointwhichcontainCH3atoneendwhileHatotherend.
Bothsideofbondcontaindifferentmasswhichisresponsibleforthevibarationofthe
molecule.
SomoleculecontaingpermanentdipolemomentshowvibrationalspectroscopyorIR
spectroscopy.
RangeofIRSpectroscop
0.8µ 2.5µ 15µ
200µ
NearIR IR FarIR
12500 4000 667
50

2. IR
Functionalgroupregion4000cm
1-
to1500cm
1-
fingerprintregion 1500cm
1-
to667cm
1-
Samplepreparation
Gaseoussamplesrequirelittlepreparationbeyondpurification,butasamplecellwitha
longpathlength(typically5-10cm)isnormallyneeded,asgasesshowrelativelyweak
absorbance’s.
Liquidsamplescanbesandwichedbetweentwoplatesofahighpuritysalt(commonly
sodium chloride,orcommonsalt,althoughanumberofothersaltssuchaspotassium
bromideorcalcium fluoridearealsoused).Theplatesaretransparenttotheinfrared
lightandwillnotintroduceanylinesontothespectra.Somesaltplatesarehighly
solubleinwater,sothesampleandwashingreagentsmustbeanhydrous(without
water
Solidsamplescanbepreparedintwomajorways.Thefirstistocrushthesamplewith
amullingagent(usuallynujol)inamarbleoragatemortar,withapestle.Athinfilm of
themullisappliedontosaltplatesandmeasured.
Thesecondmethodistogrindaquantityofthesamplewithaspeciallypurifiedsalt
(usuallypotassium bromide)finely(toremovescatteringeffectsfrom largecrystals).
Thispowdermixtureisthencrushedinamechanicaldiepresstoform atranslucent
pelletthroughwhichthebeamofthespectrometercanpass
Itisimportanttonotethatspectraobtainedfromdifferentsamplepreparationmethods

3. willlookslightlydifferentfrom eachotherduetodifferencesinthesamples'physical
states
DifferenttypeofVibrationalmode
Therearegenerallytwotypeofvibrationalmode
Stretchingisrelatedtobondlength,andtherearetwotypeofstretching
Symmetricstretchingwhenbothsideatommoveinsamedirectionasboth
movetowardcentralatomorbothsideatommoveawayfromcentralatom,
asaresultbothbondlengthincreasesordecreasetogether.
Asymmetricstretchingwhenbothsideatommovesoppositeofeachother
onemovetowardcentralatomwhileothermoveawayfromcentralatom
Bendingitisrelatedtobondangle,therearefourtypeofbendingmode
ScissoringWhenbothsideatommoveinsamedirectionasaresultbond
angleremainconstant
Rockingwhenbothsideatommoveinoppositedirectionofeachother
Waggingwhenbothsideatommoveinsamedirectionofplane,aboveor
belowtheplane
Twistingwhenbothsideatommoveinoppositedirectionofplaneone
moveabovetheplaneotherbelowtheplane.

4. TheOriginofGroupFrequencies
Animportantobservationmadebyearlyresearchersisthatmanyfunctionalgroup
absorbinfraredradiationataboutthesamewavenumber,regardlessofthestructureof
therestofthemolecule.Forexample,C-Hstretchingvibrationsusuallyappearbetween
3200and2800cm-1andcarbonyl(C=O)stretchingvibrationsusuallyappearbetween
1800and1600cm-1.Thismakesthesebandsdiagnosticmarkersforthepresenceofa
functionalgroup in a sample.These types ofinfrared bands are called group
frequenciesbecausetheytellusaboutthepresenceorabsenceofspecificfunctional
groupsinasample

5. IRSpectroscopyTutorial:Alkanes
ThespectraofsimplealkanesarecharacterizedbyabsorptionsduetoC–Hstretching
andbending(theC–Cstretchingandbendingbandsareeithertooweakoroftoolowa
frequencytobedetectedinIRspectroscopy).Insimplealkanes,whichhaveveryfew
bands,eachbandinthespectrumcanbeassigned.
C–Hstretchfrom3000–2850cm-1
C–Hbendorscissoringfrom1470-1450cm-1
C–Hrock,methylfrom1370-1350cm-1
C–Hrock,methyl,seenonlyinlongchainalkanes,from725-720cm-1
TheIRspectrumofoctaneisshownbelow.Notethestrongbandsinthe3000-2850cm-
1regionduetoC-Hstretch.TheC-Hscissoring(1470),methylrock(1383),andlong-
chainmethylrock(728)arenotedonthisspectrum.Sincemostorganiccompounds
havethesefeatures,theseC-H vibrationsareusuallynotnotedwheninterpretinga
routineIRspectrum
Theregionfrom about1300-900cm-1iscalledthefingerprintregion.Thebandsinthis
regionoriginateininteractingvibrationalmodesresultinginacomplexabsorption
pattern.Usually,thisregionisquitecomplexandoftendifficulttointerpret;however,

6. eachorganiccompoundhasitsownuniqueabsorptionpattern(orfingerprint)inthis
regionandthusanIRspectrum beusedtoidentifyacompoundbymatchingitwitha
sampleofaknowncompound
IRSpectroscopyTutorial:Alkenes
Alkenesarecompoundsthathaveacarbon-carbondoublebond,–C=C–.Thestretching
vibrationoftheC=Cbondusuallygivesrisetoamoderatebandintheregion1680-1640
cm-1.
Stretchingvibrationsofthe–C=C–Hbondareofhigherfrequency(higherwavenumber)than
thoseofthe–C–C–Hbondinalkanes.ThisisaveryusefultoolforinterpretingIRspectra:Only
alkenesandaromaticsshowaC-Hstretchslightlyhigherthan3000cm-1.Compoundsthatdo
nothaveaC=CbondshowC-Hstretchesonlybelow3000cm-1.
C=Cstretchfrom1680-1640cm-1
=C–Hstretchfrom3100-3000cm-1

7. =C–Hbendfrom1000-650cm-1
TheIRspectrum of1-octeneisshownbelow.Notethebandgreaterthan3000cm-1for
the=C–H stretchandtheseveralbandslowerthan3000cm-1for–C–H stretch
(alkanes).TheC=Cstretchbandisat1644cm-1.BandsforC–Hscissoring(1465)and
methylrock(1378)aremarkedonthisspectrum;inroutineIRanalysis,thesebandsare
notspecifictoanalkeneandaregenerallynotnotedbecausetheyarepresentinalmost
allorganicmolecules(andtheyareinthefingerprintregion).Thebandsat917cm-1and
1044cm-1areattributedto=C&Mac173;H
TheOrginsofPeakWidths
Ingeneral,thewidthofinfraredbandsforsolidandliquidsamplesisdeterminedbythe
numberofchemicalenvironmentswhichisrelatedtothestrengthofintermolecular
interactionssuchashydrogenbonding.Figure1.showshydrogenbondinwater
moleculesandthesewatermoleculesareindifferentchemicalenvironments.Because
thenumberandstrengthofhydrogenbondsdifferswithchemicalenvironment,the
forceconstantvariesandthewavenumberdiffersatwhichthesemoleculesabsorb

8. infraredlight.
Inanysamplewherehydrogenbondingoccurs,thenumberandstrengthof
intermolecularinteractionsvariesgreatlywithinthesample,causingthebandsinthese
samplestobeparticularlybroad.Thisisillustratedinthespectraofethanol(Fig7)and
hexanoicacid(Fig11).Whenintermolecularinteractionsareweak,thenumberof
chemicalenvironmentsissmall,andnarrowinfraredbandsareobserved.