1. Infrared Spectroscopy
By: Bijaya Kumar Uprety

2. Introduction
•Thetwoatomsjoinedtogetherbyachemicalbond(maybesingle,doubleortriplebond),macroscopicallycanbecomposedastwoballsjoinedbyaspring.
•Theapplicationofaforcelike(i)stretchingofoneorboththeballs(atoms)awayfromeachotherorclosertoeachother(ii)bendingofoneoftheatomseitherverticallyorhorizontallyandthenreleaseoftheforceresultsinthevibrationsonthetwoballs(atoms).
•Thesevibrationsdependonthestrengthofthespringandalsothemode(stretchingorbending)inwhichtheforceisbeingapplied.
•Similarly,atordinarytemperatures,organicmoleculesareinaconstantstateofvibrations,eachbondhavingitscharacteristicstretchingandbendingfrequencies.
•Wheninfraredlightradiationsbetween4000-400cm-1(theregionmostconcernedtoanorganicchemist)arepassedthroughasampleofanorganiccompound, someoftheseradiationsareabsorbedbythesampleandareconvertedintoenergyofmolecularvibrations.Theotherradiationswhichdonotinteractwiththesamplearetransmittedthroughthesamplewithoutbeingabsorbed.Theplotof% transmittanceagainstfrequencyiscalledtheinfraredspectrumofthesampleorcompound.

3. Continue…
•Thisstudyofvibrationsofbondsbetweendifferentatomsandvariedmultiplicitieswhichdependingontheelectronegativity,massesoftheatomandtheirgeometryvibrateatdifferentbutspecifiedfrequencies;iscalledinfraredspectroscopy.
•Thepresenceofsuchcharacteristicvibrationalbandsinaninfraredspectrumindicatesthepresenceofthesebondsinthesampleunderinvestigation.
Principle:
IRspectroscopyisbasedupontheprinciplethat,whenacompoundisexposedtoIRradiations,itselectivelyabsorbstheradiationsresultinginvibrationofthemoleculesofthecompound.Itresultsincloselypackedabsorptionbandswhicharecharacteristictothefunctionalgroupsandbondspresentinthecompound.ThusanIRspectrumofcompoundisconsideredasthefingerprintforitsidentification.

4. •Theinfraredportionoftheelectromagneticspectrumisusuallydividedintothreeregions;thenear-,mid- andfar-infrared,namedfortheirrelationtothevisiblespectrum.
•Thehigher-energynear-IR,approximately14000– 4000cm−1(0.8–2.5μmwavelength)canexciteovertoneorharmonicvibrations.
•Themid-infrared,approximately4000–400cm−1(2.5– 25μm)maybeusedtostudythefundamentalvibrationsandassociatedrotational- vibrationalstructure.
•Thefar-infrared,approximately400–10cm−1(25– 1000μm),lyingadjacenttothemicrowaveregion,haslowenergyandmaybeusedforrotationalspectroscopy.

5. Hooke’s law and Absorption of radiations
•
•Therefore,inprinciple,eachabsorptionofradiationintheinfraredregionisquantizedandshouldappearassharpline.However,eachvibrationaltransitionwithinthemoleculeisassociatedwithnumberofrotationalenergychangesandthusappearsascombinationofvibrational-rotationalbands.
•Theanalogyofachemicalbondwithtwoatomslinkedthroughaspringcanbeusedtorationalizeseveralfeaturesoftheinfraredspectroscopy.
•TheapproximationtovibrationfrequencyofabondcanbemadebytheapplicationofHooke’slaw.InHooke’slaw,twoatomsandtheirconnectingbondaretreatedasasimpleharmonicoscillatorcomposedoftwomassesjoinedbyaspringandfrequencyofvibrationisstatedas

7. •Therefore,thevibrationalfrequencyofabondwouldincreasewiththedecreaseinreducedmassofthesystem.ItimpliesthatC-HandO-HstretchingabsorptionsshouldappearathigherfrequenciesthanC-CandC-Ostretchingfrequencies.Further,inparallelwiththegeneralknowledgethatthestretchingofthespringrequiresmoreenergythantobendit,thestretchingabsorptionofabandalwaysappearathigherenergythanthebendingabsorptionofthesameband.
•TheHooke’slawcanbeusedtotheoreticallycalculatetheapproximatestretchingfrequencyofabond.ThevalueofKisapproximately5x105dyne/cmforsinglebondsandapproximatelytwoandthreetimesthisvalueforthedoubleandtriplebonds,respectively.
•LetuscalculatetheapproximatefrequencyoftheC-Hstretchingvibrationfromthemassesofcarbonandhydrogen.

8. IR Frequency Range
•Infraredradiationspansasectionoftheelectromagneticspectrumhavingwavenumbersfromroughly13,000to10cm–1,orwavelengthsfrom0.78to1000μm.Itisboundbytheredendofthevisibleregionathighfrequenciesandthemicrowaveregionatlowfrequencies.
•IRabsorptionpositionsaregenerallypresentedaseitherwavenumbersorwavelengths(l).
•Wavenumberdefinesthenumberofwavesperunitlength.Thus,wavenumbersaredirectlyproportionaltofrequency,aswellastheenergyoftheIRabsorption. Thewavenumberunit(cm–1,reciprocalcentimeter)ismorecommonlyusedinmodernIRinstrumentsthatarelinearinthecm–1scale.
•Inthecontrast,wavelengthsareinverselyproportionaltofrequenciesandtheirassociatedenergy.
•Wavenumbersandwavelengthscanbeinterconvertedusingthefollowingequation:

9. •ThefarIRrequires[400–10cm−1(25–1000μm)]theuseofspecializedopticalmaterialsandsources.Itisusedforanalysisoforganic,inorganic, andorganometalliccompoundsinvolvingheavyatoms(massnumberover19).Itprovidesusefulinformationtostructuralstudiessuchasconformationandlatticedynamicsofsamples.
•NearIR[14000–4000cm−1(0.8–2.5μmwavelength)]needsminimalornosamplepreparation.Itoffershigh-speedquantitativeanalysiswithoutconsumptionordestructionofthesample.ItsinstrumentscanoftenbecombinedwithUV-visiblespectrometerandcoupledwithfiberopticdevicesforremoteanalysis.NearIRspectroscopyhasgainedincreasedinterest,especiallyinprocesscontrolapplications.
•However,themostfrequentlyusedregionismidIRregionwhichliesbetween4000and400cm–1.

10. Region of IR Spectrum
•A molecule absorbs IR radiations of various wavelengths depending upon the nature of groups or bonds present in it and gives characteristic absorption bands.
•The presence of characteristic absorption band indicates the presence of a particular functional group.
•There are two general regions in the infrared spectrum, namely :
(a)Groupfrequencyregion/FunctionalGroupRegion:havingawavenumberfrom4000-1500cm–1.Here,thestretchingandbendingvibrationalbandsassociatedwithspecificstructuralorfunctionalgroupsareobservedfrequently.
(b)Fingerprintregion:havingawavenumberfrom1500-400cm–1.Here,thevibrationalmodesdependsolelyandstronglyontherestofthemolecule.Example:TheC—Cstretchingfrequencydependslargelyonwhatelseisbondedtothecarbonatoms.Itisinterestingtoobserveherethatthisparticularregionofthespectrumisdenselypopulatedwithbands.Asweknowthatnotwo‘fingerprints’couldbeidenticalinhumanbeings,exactlyinasimilarmannernotwocompoundsmayhavethesame‘fingerprintregion’.Thus,eachandeverymoleculeessentiallygivesrisetoauniquespectrumwhichoffersacharacteristicfeatureofthesame.

11. The primary regions of the IR spectrum

12. Criteria for Absorption of IR radiation
(i)DipoleMoment:
•Weknowthatatordinarytemperature,moleculesareinconstantstateofvibrations. Thechangeindipolemomentduringvibrationofthemoleculeproducesastationaryalternatingelectricfield.
•Whenthefrequencyofincidentelectromagneticradiationsisequaltothealternatingelectricfieldproducedbychangesinthedipolemoment,theradiationisabsorbedandvibrationallevelsofthemoleculeareexcited.
•Onceinthevibrationallyexcitedstate,themoleculescanloosetheextraenergybyrotational,collisionortranslationalprocessesetc.andcomebacktogroundstate.
•Therefore,onlythosevibrationswhichresultinarhythmicalchangeinthedipolemomentofthemoleculeabsorbinfraredradiationsandaredefinedasIRactive.
•TheotherswhichdonotundergochangeindipolemomentofthemoleculeareIRinactivee.g.thestretchingofasymmetricallysubstitutedbond,vizC≡Cinacetyleneandsymmetricalstretchingincarbondioxide,alinearmolecule,producenochangeinthedipolemomentofthesystemandthesevibrationscannotinteractwithinfraredlightandareIRinactive.
•Ingeneral,thefunctionalgroupsthathaveastrongdipolegiverisetostrongabsorptionbandsintheIR.

13. (ii)WavelengthofIncidentRadiation:Theabsorptionofradiationsbythemoleculeoccursonlywhenthefrequencyofincidentradiationisequivalenttonaturalfrequencyofvibrationofthepartofthemolecule.Thepartofthemoleculecanbeanatomorgroupofatom.Afterabsorptionofcharacteristicradiations,themoleculevibrateswithgreateramplitudeduetoabsorbedIRenergy.

14. Modes of Vibration
•Allmoleculesarecontinuallyvibrating.Thesevibrationareoftwotypes:
A.Stretchingvibrations:Inthistypeofvibration,thedistancebetweenthetwoatomsincreasesordecreases,buttheatomsremaininthesamebondaxis.Thisisduetochangeinbondlength.Thestretchingvibrationsmaybefurthersub-dividedintotwocategories,namely:
(a)SymmetricalStretching:Inthiscase,thetwoatoms(AandB)eithermovetowardsorawayfromthecentralatom,withoutchangeinbondaxisorbondangle.
(b)AsymmetricalStretching:Inthisinstance,thetwoatomsAandBmovewithrespecttocentralatomsuchthatonemovesawayandtheothermovestowardsthecentralatomC.
B.BendingVibration:Thesetypeofvibrationinvolvesmovementofatomswhichareattachedtoacommoncentralatom,suchthatthereischangeinbondaxisandbondangleofeachindividualatomwithoutchangeintheirbondlength.Bendingvibrationscantakeplaceeitherin-planeorout-of-plane.

15. (i) In-Plane Bending Vibrations
These are two types :
(a) Scissoring or Symmetrical Bending : In these type of vibration, in plane bending of atoms occur wherein they move back and forth.
(b) Rocking : In this case, in plane bending of atoms occur wherein they swing back and forth with respect to the central atom.
(ii) Out-of Plane Bending Vibrations
These are also of two kinds, namely :
(a) Wagging : In these type of vibrations, out of plane bending of atoms occur wherein they oscillate back and forth.
(b) Twisting : In these types of vibrations, out of plane bending of atoms occur wherein they rotate around the bond which joins the central atom C of molecule.

16. •Eachbendingandstretchingvibrationofabondinamoleculeoccurswithacharacteristicfrequency.
•Whenacompoundisbombardedwithradiationofafrequencythatexactlymatchesthefrequencyofoneofitsvibrations,themoleculewillabsorbenergy.
•Determiningthewavenumbersoftheenergyabsorbedbyaparticularcompound,wecantellwhatkindsofbondsithas.
•The bending (or deformation) vibrations generally require less energy and take place at longer-wavelength than the corresponding stretching vibrations.
•In contrast, the stretching vibrations are observed to occur with respect to their corresponding bond strengths.
Note:
v(asymmetricstretching)>v(symmetricstretching)>v(bending)

17. •Wheneveraverysmallprotonlike:C—H;O—H;orN—Hisinvolvedinasinglebond,thestretchingvibrationsnormallytakeplaceatmuchhigherfrequencyi.e., 3700-2630cm–1(or2.7-3.8μ).Itis,however,interestingtonotethatO—Hbondabsorbsat2.8μ(or3570cm–1),whereasO—Dbondabsorbsat3.8μ(or2630cm–1).Inthisspecificcase,thestrengthsofthetwobondsaremoreorlessthesame,butthemassofoneatomisalmostdoubled.(i.edeuteriumisalmostdoublethemassofhydrogen)
•Moleculeswithlargenumberofatomspossessalargenumberofvibrationalfrequencies.Foranon-linearmoleculewithnatoms,thenumberoffundamentalvibrationalmodesis(3n-6);linearmoleculeshave3n-5fundamentalvibrationalmodes.Therefore,water-anon-linearmoleculetheoreticallypossesses3fundamentalvibrations–twostretchingandonebending;whereascarbondioxide-alinearmoleculepossess4fundamentalabsorptionbandsinvolvingtwostretchingandtwobendingmodes.

18. For animation: http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec1.htm

19. Factors influencing vibrational Frequencies
•There are a number of factors that influence the precise frequency of a molecular vibration, namely :
(a) Vibrational coupling,
(b) Hydrogen bonding,
(c) Electronic Effects, and
(d) Field Effects.
a.Vibrational coupling:
•Vibrational coupling involves interaction between two similar or different vibrating groups. Vibrational coupling depends upon various factors such as:
1.When two atoms are bonded to a common central atom, the stretching vibrations of these atoms show strong coupling.
2.Coupling between bending vibrations is seen only when the two vibrating groups are separated by a common bond.
3.Coupling is maximum when the vibrating groups have individual energies which are approximately equal in magnitudes.
4.If the vibrating groups are separated by two or more bonds, negligible or no coupling takes place.

20. •Example1:VibrationcouplingcanbebestexplainedbytakinganexampleoftriatomicmoleculesuchasCO2.AsCO2isalinearmolecule,thenumberofmodesofvibrationsforitisgivenby;
3n-5=3x3-5=4.ThusnumberofpeaksshouldbefourintheIRspectrum(twostretchingandtwobending)
•Buthereonlytwopeaksappearthatisanti-symmetricandbendingpeaks.
•Anti-symmetricpeaksappearat2350cm-1.
•Heresymmetricpeakdoesn’tappearbecausethereisnochangeindipolemoment.ThereforeitisIRinactive.
•Andinsteadoftwobendingpeakonlyonebendingpeakappearsbecauseenergyofabsorptionissame.
•Example2:Anothere.g.isofAcetaldehyde.Heren=7andmoleculeisnon-linearsothenumberofpeaksaccordingtoformula3n-6shouldbe15butonlyfivepeaksappearinthespectraduetovibrationalcoupling.

21. (b)HydrogenBonding
•HydrogenbondingisatypeofelectrostaticinteractionbetweenmoleculesthathavehydrogenatomandelectronegativeatomslikeF,N, Oetc.
•Compoundscontainingprotondonorgroupssuchascarbonyl,hydroxylamine(NH2-OH),amide(CO-NH2),etccanbeinvolvedinintra-orintermolecularhydrogenbondinginthepresenceofprotonacceptors, e.g.O,N,halogens,C=C.
•ThestiffnessoftheX—Hbondistherebylessened,resultinginaloweringofthestretchingfrequency,andthebandbroadensandoftenintensifies.
•Conversely,thefrequencyofthebendingmodeisraisedbuttheeffectismuchlesspronounced.
•Intermolecularhydrogenbondingissuppressedatelevatedtemperaturesbutitisfavouredbyahighsoluteconcentration.
•Intramolecularhydrogenbondingisalsoreducedatelevatedtemperaturesbutitisunaffectedbychangesinsoluteconcentration. Theseeffectsareparticularlysignificantinthespectraofalcohols, phenols,carboxylicacidsandamines.

22. •GroupssuchashydroxylandamineshowscharacteristicvibrationalfrequencieswhenexposedtoIRradiations.However,thesefrequenciesarealteredwhenthecompoundcontainingthesegroupsexhibitshydrogenbonding.
•E.g.Hydrogenbondinginanalcohols(-OH)isstrongerthanthatinamines(-NH2)because,oxygenofalcoholismoreelectronegativethannitrogenofamines.
•Thus,vibrationalfrequencyof(-OH)groupisloweredmoreinIRspectrumofalcoholswhencomparedto(-NH2)groupofamines.
•Strengthofhydrogenbondingisinverselyproportionaltothechangeinfundamentalvibrationalfrequencyofthecompoundinvolvedinbonding.i.e.ifthehydrogenbondingisstrong,then, thefundamentalvibrationalfrequencyofthegroupinvolvedinbondingisloweredandtheabsorptionbandobtainedisbroaderandmoreintenseandviceversa.

23. (c)ElectronicEffect:
•Dependsonthepresenceofsubstituent.
•Inorganicmolecules,theelectrondisplacementtakesplacebyvariousmechanismsuchasConjugationeffect,mesomericeffectandinductiveeffects.
•ThesemechanisminfluencesthevibrationfrequenciesinIRspectroscopy.
1.Conjugationeffect:
•ItisobservedtolowerthefrequencyofbothC=Cstr.andC=Ostr.
•In II, lowering of band order in the C = C bond is observed due to conjugation effect whereby the stretching vibration frequency is decreased by 40-50 cm–1(compare I and II above).

24. •Inasimilarmanner,inVdelocalizationofπ-electronsbetweenC=Oandthebenzeneringenhancesthedouble-bondcharacterofthebondjoiningtheC=Otothering.ItultimatelyleadstoalowerbandorderintheC=Obondtherebydecreasingthestretchingvibrationfrequencyby20-30cm–1(compareIIIandVabove).
2.Resonanceeffect(Mesomericeffect):
•Meanssinglemoleculecanberepresentedintwoormorethantwostructuresthatdifferonlyinthearrangementofelectrons.
•Ifelectronreleasinggrouppresentincreasesindelocalizationofelectronitdecreasesthedoublebondcharacteristicincreaseinbondlengthdecreaseinbondstrengthsodecreasesvibrationalfrequency.
•Similarly,ifelectronwithdrawinggrouppresentitincreasesvibrationalfrequency.

26. 3.InductiveEffect:
•Theinductiveeffectssolelydependsuponthe‘intrinsic’tendencyofasubstituenttoeitherreleaseorwithdrawelectrons,itselectronegativityactingeitherthroughthemolecularchainorthroughspace.Thiseffectusuallyweakenssteadilywithincreasingdistancefromthesubstituent.
•Inductiveandresonancebothtypeofeffectsareexistinginmolecule.
•Finallywhichtypeofeffectisshownbyamoleculedependsonwhicheffectisprominent.
e.g.
i.Amides:(R-C-NH2):Containsmoreelectronreleasingmoiety(NH2) thusenhancedresonanceeffectsodecreasedvibrationalfrequency. Fig(q)
ii.Acylchloride(R-CO-Cl):C=Oiselectronwithdrawingsodecreaseindelocalizationofelectronincreasedbondstrengthincreasevibrationalfrequency.Fig(r)

28. 4. Field Effect

29. Instrumentation
•Theimportantfeaturesofaninfraredspectrophotometerareasfollows:
(i)Infraredsources,
(ii)Monochromators,
(iii)Detectors,and
(iv)ModeofOperation
InfraredSources
•Themostcommoninfraredsourcesareelectricallyheatedrodsofthefollowingtypes:
(a)FusedmixturesoftheoxidesofZirconium(Zr),Yttrium(Y),Erbium(Er)etc., alsoknownas‘NernstGlower’,
(b)SiliconCarbide‘Globar’,and
(c)Variousceramic(clay)materials.
•Itisquiteevidentthattheinfraredoutputfromallthesedifferentsourcesinvariablyvariesinintensityoveradefinitefrequencyrange,therefore,acompensatingvariableslitisusuallyprogrammedtooperateinunisonwiththescanningovertheindividualfrequencies.

30. Monochromators
•Threetypesofsubstancesarenormallyemployedasmonochromators,namely:
(i)MetalHalidePrisms:Variousmetalhalideprisms,suchas:KBr(12- 25μm),LiF(0.2-6μm)andCeBr(15-38μm)havebeenusedearlier, buttheyhavebecomemoreorlessobsolescentnowadays.
(ii)NaClPrism(2-15μm):Sodiumchlorideprismareofuseforthewholeoftheregionfrom4000-650cm–1.First,itofferslowresolutionat4000-2500cm–1,andsecondly,becauseofitshygroscopicnaturetheopticshavegottobeprotectedat20°Cabovetheambienttemperature.
(iii)Gratings:Ingeneral,gratingsarecommonlyemployedinthedesignoftheinstrumentsandofferbetterresolutionathigherfrequencythantheprisms.Theyoffermuchbetterresolutionatlowfrequency,viz.,typicalrulingsare240linespernmforthe4000- 1500cm–1regionand120linespernmforthe1500-650cm–1region.

31. Detectors
•Therearethreedifferenttypesofdetectorsthatareusedintheinfraredregion:
(a)Thermocouples(orThermopiles):Theunderlyingprincipleofathermocoupleisthatiftwodissimilarmetalwiresarejoinedheadtotail,thenadifferenceintemperaturebetweenheadandtailcausesacurrenttoflowinthewires.Intheinfraredspectrophotometerthiscurrentshallbedirectlyproportionaltotheintensityofradiationfallingonthethermocouple.Hence,thethermocouplesareinvariablyemployedintheinfraredregion,andtohelpinthecompleteabsorptionof‘availableenergy’the‘hot’junctionorreceiverisnormallyblackened.
(b)GolayDetector:Inthisspecificinstancetheabsorptionofinfraredradiationaffordsexpansionofaninertgasinacell-chamber.Theheatproducedduetoabsorptionofradiationcausesthegastoexpandandthusdeformtheflexiblesilvereddiaphragmwhichactsasmirror.Thedeformationofdiaphragmisamplifiedbyplacingalampinthedetector.Fromthelamp,lightismadetofallonthediaphragmwhichinturnreflectsitontothephototube.Whendiaphragmisatrestnolightpassesthroughitbutwhendiaphragmflexesvaryingamountoflightreachesphototube.Thecurrentfromthephototubeisdirectlyproportionaltotheincidentradiation.

32. Figure: Golaydetector.

33. (c)Bolometers:Thesearebasedontheprinciplethatmakeuseoftheincreaseinresistanceofametalwithincreaseintemperature.Forinstance,whenthetwoplatinumfoilsareappropriatelyincorporatedintoaWheatstonebridge,andradiationisallowedtofallononeofthefoil(knownasindicatorstrip),achangeintheresistanceisobservedultimately.Thiscausesanout-of-balancecurrentthatisdirectlyproportionaltotheincidentalradiation.Justlikethethermocouples,theyareusedintheinfraredregion.
TypesofIRspectrometer
Theinstrumentthatdeterminestheabsorptionspectrumforacompoundiscalledaninfraredspectrometeror,moreprecisely,aspectrophotometer.Twotypesofinfraredspectrometersareincommonuseintheorganiclaboratory:dispersiveandFouriertransform(FT) instruments.Bothofthesetypesofinstrumentsprovidespectraofcompoundsinthecommonrangeof4000to400cm−1.Althoughthetwoprovidenearlyidenticalspectraforagivencompound,FTinfraredspectrometersprovidetheinfraredspectrummuchmorerapidlythanthedispersiveinstruments.

34. Dispersive Infrared Spectrometers
•Figure2.3aschematicallyillustratesthecomponentsofasimpledispersiveinfraredspectrometer.Theinstrumentproducesabeamofinfraredradiationfromahotwireand,bymeansofmirrors,dividesitintotwoparallelbeamsofequal-intensityradiation.Thesampleisplacedinonebeam,andtheotherbeamisusedasareference.Thebeamsthenpassintothemonochromator,whichdisperseseachintoacontinuousspectrumoffrequenciesofinfraredlight.Themonochromatorconsistsofarapidlyrotatingsector(beamchopper) thatpassesthetwobeamsalternatelytoadiffractiongrating(aprisminolderinstruments).Theslowlyrotatingdiffractiongratingvariesthefrequencyorwavelengthofradiationreachingthethermocoupledetector.Thedetectorsensestheratiobetweentheintensitiesofthereferenceandsamplebeams.Inthisway,thedetectordetermineswhichfrequencieshavebeenabsorbedbythesampleandwhichfrequenciesareunaffectedbythelightpassingthroughthesample. Afterthesignalfromthedetectorisamplified,therecorderdrawstheresultingspectrumofthesampleonachart.Itisimportanttorealizethatthespectrumisrecordedasthefrequencyofinfraredradiationchangesbyrotationofthediffractiongrating.Dispersiveinstrumentsaresaidtorecordaspectruminthefrequencydomain.

35. •Notethatitiscustomarytoplotfrequency(wavenumber,cm−1)versuslighttransmitted,notlightabsorbed.Thisisrecordedaspercenttransmittance(%T)becausethedetectorrecordstheratiooftheintensitiesofthetwobeams,and
%Transmittance=Is/Irx100
•whereIstheintensityofthesamplebeam,andIristheintensityofthereferencebeam.Inmanypartsofthespectrum,thetransmittanceisnearly100%,meaningthatthesampleisnearlytransparenttoradiationofthatfrequency(doesnotabsorbit).Maximumabsorptionisthusrepresentedbyaminimumonthechart.Evenso,theabsorptionistraditionallycalledapeak.Thechemistoftenobtainsthespectrumofacompoundbydissolvingitinasolvent.
•Thesolutionisthenplacedinthesamplebeam,whilepuresolventisplacedinthereferencebeaminanidenticalcell.Theinstrumentautomatically“subtracts”thespectrumofthesolventfromthatofthesample.Theinstrumentalsocancelsouttheeffectsoftheinfrared- activeatmosphericgases,carbondioxideandwatervapor,fromthespectrumofthesample(theyarepresentinbothbeams).Thisconveniencefeatureisthereasonmostdispersiveinfraredspectrometersaredouble-beam(sample+reference)instrumentsthatmeasureintensityratios;sincethesolventabsorbsinbothbeams,itisinbothtermsoftheratioIs/Irandcancelsout.Ifapureliquidisanalyzed(nosolvent),thecompoundisplacedinthesamplebeam,andnothingisinsertedintothereferencebeam.Whenthespectrumoftheliquidisobtained,theeffectsoftheatmosphericgasesareautomaticallycanceledsincetheyarepresentinbothbeams

37. B. Fourier Transform Spectrometers
•Themostmoderninfraredspectrometers(spectrophotometers)operateonadifferentprinciple.Thedesignoftheopticalpathwayproducesapatterncalledaninterferogram.Theinterferogramisacomplexsignal,butitswave-likepatterncontainsallthefrequenciesthatmakeuptheinfraredspectrum.
•Aninterferogramisessentiallyaplotofintensityversustime(atime- domainspectrum).However,achemistismoreinterestedinaspectrumthatisaplotofintensityversusfrequency(afrequency-domainspectrum).AmathematicaloperationknownasaFouriertransform(FT) canseparatetheindividualabsorptionfrequenciesfromtheinterferogram,producingaspectrumvirtuallyidenticaltothatobtainedwithadispersivespectrometer.ThistypeofinstrumentisknownasaFouriertransforminfraredspectrometer,orFT-IR.
•TheadvantageofanFT-IRinstrumentisthatitacquirestheinterferograminlessthanasecond.Itisthuspossibletocollectdozensofinterferogramsofthesamesampleandaccumulatetheminthememoryofacomputer. WhenaFouriertransformisperformedonthesumoftheaccumulatedinterferograms,aspectrumwithabettersignal-to-noiseratiocanbeplotted.AnFT-IRinstrumentisthereforecapableofgreaterspeedandgreatersensitivitythanadispersioninstrument.

38. •AschematicdiagramofanFT-IRisshowninFigure2.3b.
•TheFT-IRusesaninterferometertoprocesstheenergysenttothesample.Intheinterferometer,thesourceenergypassesthroughabeamsplitter,amirrorplacedata45°angletotheincomingradiation,whichallowstheincomingradiationtopassthroughbutseparatesitintotwoperpendicularbeams,oneundeflected,theotherorientedata90°angle.
•Onebeam,theoneorientedat90°inFigure2.3b,goestoastationaryor“fixed”mirrorandisreturnedtothebeamsplitter.Theundeflectedbeamgoestoamovingmirrorandisalsoreturnedtothebeamsplitter.
•Themotionofthemirrorcausesthepathlengththatthesecondbeamtraversestovary.Whenthetwobeamsmeetatthebeamsplitter,theyrecombine,butthepathlengthdifferences(differingwavelengthcontent) ofthetwobeamscausebothconstructiveanddestructiveinterferences.
•Thecombinedbeamcontainingtheseinterferencepatternsiscalledthe
interferogram.Thisinterferogramcontainsalloftheradiativeenergycomingfromthesourceandhasawiderangeofwavelengths.

39. •Theinterferogramgeneratedbycombiningthetwobeamsisorientedtowardthesamplebythebeamsplitter.Asitpassesthroughthesample,thesamplesimultaneouslyabsorbsallofthewavelengths(frequencies)thatarenormallyfoundinitsinfraredspectrum.
•Themodifiedinterferogramsignalthatreachesthedetectorcontainsinformationabouttheamountofenergythatwasabsorbedateverywavelength(frequency).
•Thecomputercomparesthemodifiedinterferogramtoareferencelaserbeamtohaveastandardofcomparison.Thefinalinterferogramcontainsalloftheinformationinonetime-domainsignal,asignalthatcannotbereadbyahuman.
•AmathematicalprocesscalledaFouriertransformmustbeimplementedbycomputertoextracttheindividualfrequenciesthatwereabsorbedandtoreconstructandplotwhatwerecognizeasatypicalinfraredspectrum.Computer-interfacedFT-IRinstrumentsoperateinasingle-beammode.

40. •Toobtainaspectrumofacompound,thechemistfirstobtainsaninterferogramofthe“background,”whichconsistsoftheinfrared- activeatmosphericgases,carbondioxideandwatervapor(oxygenandnitrogenarenotinfraredactive).
•TheinterferogramissubjectedtoaFouriertransform,whichyieldsthespectrumofthebackground.Thenthechemistplacesthecompound(sample)intothebeamandobtainsthespectrumresultingfromtheFouriertransformoftheinterferogram.
•Thisspectrumcontainsabsorptionbandsforboththecompoundandthebackground.Thecomputersoftwareautomaticallysubtractsthespectrumofthebackgroundfromthesamplespectrum,yieldingthespectrumofthecompoundbeinganalyzed. Thesubtractedspectrumisessentiallyidenticaltothatobtainedfromatraditionaldouble-beamdispersiveinstrument.

41. PREPARATION OF SAMPLES FOR INFRARED SPECTROSCOPY
•Todeterminetheinfraredspectrumofacompound,onemustplacethecompoundinasampleholder,orcell.Ininfraredspectroscopy, thisimmediatelyposesaproblem.
•Glassandplasticsabsorbstronglythroughouttheinfraredregionofthespectrum.Cellsmustbeconstructedofionicsubstancestypicallysodiumchlorideorpotassiumbromide.
•Potassiumbromideplatesaremoreexpensivethansodiumchlorideplatesbuthavetheadvantageofusefulnessintherangeof4000to400cm−1.
•Sodiumchlorideplatesareusedwidelybecauseoftheirrelativelylowcost.Thepracticalrangefortheiruseinspectroscopyextendsfrom4000to650cm−1.
•Sodiumchloridebeginstoabsorbat650cm−1,andanybandswithfrequencieslessthanthisvaluewillnotbeobserved.Sincefewimportantbandsappearbelow650cm−1,sodiumchlorideplatesareinmostcommonuseforroutineinfraredspectroscopy.

42. •Liquids.Adropofaliquidorganiccompoundisplacedbetweenapairofpolishedsodiumchlorideorpotassiumbromideplates,referredtoassaltplates.Whentheplatesaresqueezedgently,athinliquidfilmformsbetweenthem.Aspectrumdeterminedbythismethodisreferredtoasaneatspectrumsincenosolventisused.Saltplatesbreakeasilyandarewatersoluble.Organiccompoundsanalyzedbythistechniquemustbefreeofwater.Thepairofplatesisinsertedintoaholderthatfitsintothespectrometer.
•Solids.Thereareatleastthreecommonmethodsforpreparingasolidsampleforspectroscopy.Thefirstmethodinvolvesmixingthefinelygroundsolidsamplewithpowderedpotassiumbromideandpressingthemixtureunderhighpressure.
•Underpressure,thepotassiumbromidemeltsandsealsthecompoundintoamatrix.TheresultisaKBrpelletthatcanbeinsertedintoaholderinthespectrometer.Themaindisadvantageofthismethodisthatpotassiumbromideabsorbswater,whichmayinterferewiththespectrumthatisobtained.

43. •Ifagoodpelletisprepared,thespectrumobtainedwillhavenointerferingbandssincepotassiumbromideistransparentdownto400cm−1.
•Thesecondmethod,aNujolmull,involvesgrindingthecompoundwithmineraloil(Nujol)tocreateasuspensionofthefinelygroundsampledispersedinthemineraloil.Thethicksuspensionisplacedbetweensaltplates.Themaindisadvantageofthismethodisthatthemineraloilobscuresbandsthatmaybepresentintheanalyzedcompound.Nujolbandsappearat2924,1462,and1377cm−1.
•Thethirdcommonmethodusedwithsolidsistodissolvetheorganiccompoundinasolvent,mostcommonlycarbontetrachloride(CCl4)andobserveitintheIRrangebyplacingfewdropsofsampleintheIRplates(egKBrplates).Again,aswasthecasewithmineraloil,someregionsofthespectrumareobscuredbybandsinthesolvent.Althoughitispossibletocanceloutthesolventfromthespectrumbycomputerorinstrumentaltechniques,theregionaround785cm−1isoftenobscuredbythestrongC- Clstretchthatoccursthere.

44. Advantages of Fourier transform IR over dispersive IR
•ThemultiplexorFellgett'sadvantage.Thisarisesfromthefactthatinformationfromallwavelengthsiscollectedsimultaneously.ItresultsinahigherSignal-to-noiseratioforagivenscan-timeorashorterscan-timeforagivenresolution.
•ThethroughputorJacquinot'sadvantage.Thisresultsfromthefactthat, inadispersiveinstrument,themonochromatorhasentranceandexitslitswhichrestricttheamountoflightthatpassesthroughit.Theinterferometerthroughputisdeterminedonlybythediameterofthecollimatedbeamcomingfromthesource.
•ConnesadvantageThewavenumberscaleofaninterferometerisderivedfromaHe-Ne(heliumneon)laserthatactsasaninternalreferenceforeachscan.Thewavenumberofthislaserisknownveryaccuratelyandisverystable.Asaresult,thewavenumbercalibrationofinterferometersismuchmoreaccurateandhasmuchbetterlongtermstabilitythanthecalibrationofdispersiveinstruments.

45. •NegligiblestraylightBecauseofthewayinwhichtheinterferometermodulateseachsourcewavelength.Thereisnodirectequivalentofthestraylightfoundindispersivespectrometers.
•HighResolution.
•Goodforweakabsorptionbands.
•Canstudysamplesunderhighabsorbance.
•Lesstimeforscan.
•NodiscontinuitiesBecausetherearenogratingorfilterchanges,therearenodiscontinuitiesinthespectrum.

46. Disadvantages of FT-IR
•Detectors response time needs to be fast.
•Initial cost is higher.
•Maintenance is complicated.

48. CORRELATION CHARTS AND TABLES
•Toextractstructuralinformationfrominfraredspectra,youmustbefamiliarwiththefrequenciesatwhichvariousfunctionalgroupsabsorb.Youmayconsultinfraredcorrelationtables,whichprovideasmuchinformationasisknownaboutwherethevariousfunctionalgroupsabsorb.Theabsorptioninformationispresentedintheformofachartcalledacorrelationchart.

60. C-HBendingVibrations
•Thein-planeC-Hbendingvibrationsoccurbetween1300and1000cm−1.However,thesebandsarerarelyusefulbecausetheyoverlapother,strongerabsorptionsthatoccurinthisregion.
•Theout-of-planeC-Hbendingvibrations,whichappearbetween900and690cm−1,arefarmoreusefulthanthein-planebands.Theseextremelyintenseabsorptions,resultingfromstrongcouplingwithadjacenthydrogenatoms,canbeusedtoassignthepositionsofsubstituentsonthearomaticring.