Nuclear Magnetic Resonance Spectroscopy (NMR) · PDF fileOCR Chemistry A H432 Nuclear Magnetic...

OCRChemistryAH432 NuclearMagneticResonance

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NuclearMagneticResonanceSpectroscopy(NMR)Whatisit?Aninstrumentalmethodthatgivesverydetailedstructuralinformationaboutmolecules.Itcantellus -howmanyofcertaintypesofatomamoleculecontains -wheretheseatomsarelocatedinthemoleculeItworksbecausethenucleiofcertainatomshaveapropertywecall"spin".Theirspinscanbe"up(↑)"or"down(↓)".Normallythesetwospinshavethesameenergy,butwhenthenucleiareinamagneticfield,thespinsalignwiththefield.Thespinstatewhichisalignedwiththefieldisoflowerenergythanthespinstatewhichopposesthefield,sothereisanenergygap–nucleicanbeexcited,usingapulseofradiofrequencyenergy,fromthelowerenergyspinstatetothehigherenergyspinstate,andcanrelaxbacktothelowerenergyspinstatebyreleasingenergy.Theenergiesinvolvedaretiny,soNMRinstrumentationhastobeextremelysensitive.Thecommonly-usednucleiforNMRare1H,and13CalthoughitisalsopossibletodoNMRwith31Pand19Famongstotheratoms.Carbon-13NMR–signalsarisefromthecarbonatomsinthemolecule1.1%ofallnaturallyoccurringcarbonatomsare13Catoms.Theyoccurrandomlyinanypositioninthemoleculewhereacarbonatomexists,sothesignalfromthe13CatomsgivesinformationaboutalltheCatomspresentinthesample.Ifeachcarbonnucleiwasentirelyonitsown,separatedfromtheinfluenceofanyothernuclei,itwouldproduceanNMRsignalatexactlythesameenergy.Inrealitythecarbonatomshaveotheratomsbondedaroundthem.Theelectronsintheseatomssubtlyaffectthemagneticfieldthenucleusexperiences,andthereforethesignaloccursatadifferentenergy.TheenergyofanNMRsignalismeasuredintermsofhowfarawayitis(theChemicalShift,δ)fromaknownreferencesignal,inunitsofpartspermillion(ppm).Consideramoleculeofpropan-2-ol:EachcarbonatomproducesanNMRsignal,buthowmanydifferentcarbon-atomenvironmentsarethere?Thefirstandthirdcarbonsareinidenticalenvironments,soproducesignalsatthesamechemicalshift.Thesecondcarbonwillproduceasignalatadifferentchemicalshift.WewillthereforeseetwopeaksintheNMRspectrum.

H CH

CH

CH

HH

H OH

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Usingyourdatasheetyoushouldnowbeabletopredictthenumberofpeaksinthe13Cspectrumofpropan-1-olandsketchits13CNMRspectrum.Incarbon-13NMRthesizeofthepeaksDOESNOTtellusanythinguseful.(Thisisdifferentfromin1HNMR(protonNMR)whichwewilllookatnext.Thedatasheetshowswhichchemicalshiftvaluestoexpectforcarbonatomsindifferentenvironments–thisisusedtodecidewhichNMRsignalscorrespondtowhichcarbonnucleiinthemolecule.NoticehowthemoreelectronegativeatomsbondedtotheCatomincreaseitschemicalshift.Thesolvent,andconcentrationofthemoleculebeingsampledcausethechemicalshiftstovaryslightlyalso. Environment Approx.chemicalshift(δ)inppm C–C(aliphatic) 5–55 C–Cl,C–Br 30–70 C–N 35–60 C–O 50–70 C=C 115–140 C–C(aromatic) 110–165 C=OalsobondedtoOorN 160–185 C=O 190–220 (butcheckagainstA2datasheet)Practice:1:Usingthedatasheet,predictwhatthecarbon-13NMRspectrumofpropan-1-olwilllooklike. [Ans:3peaks,C-Oat65ppmC-Cat25and10ppm]2:Whatcanyoutellabouttheketonewhichproducedthis13CNMRspectrum?Canyouidentifytheketonefromthisdata? Peakat200ppm Peakat50ppm

δ (ppm) 60 40 20 0

Absorbance

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Peakat15ppm[Ans: Thereare3distinctprotonenvironments TheC=Ocarbonshowsupat200ppm TheothertwopeaksaretwodifferentaliphaticC-Cenvironments -soitcan'tbepropanone(only2environments) -itcan'tbebutanone(4differentenvironments) -itcan'tbehexanoneoranythingbigger–toomanyenvironments -pentan-2-onehas4differentCenvironments -butpentan-3-onehasexactly3differentenvironments]3:AnalcoholwithformulaC3H7OHwasoxidized.The13CNMRspectrumoftheproductshowedthreepeaks:at170ppm,45ppmand25ppm.Usingthedatatableofchemicalshifts,whatcanwesayabouttheproductandaboutthealcohol?Ans: Thepeakat170ppmcorrespondstothecarboninCOOHratherthaninC=Oso theproductislikelytobepropanoicacid. Thetwootherpeaks(aliphaticC-C)confirmthattheproductisnotpropanone (onlyonealiphaticC-Cenvironment). Wecanthereforebesurethattheoxidizedalcoholwasprimary–propan-1-ol.MoreaboutChemicalShiftNMRspectraarecalibratedusingtheδscale,inpartspermillion.ThisisbasedonthefrequenciesusedbytheNMRspectrometer.Forexample,amedium-sizedNMRinstrumentmaybereferredtoasa200MHzinstrument,thisreferstothefrequencyoftherfenergyneededtoexcitethenucleiwhentheyareinthemagneticfieldofthethatinstrument.Achemicalshiftof1partpermillionmeansashiftof200Hz(onemillionth)inthefrequencyatwhichaspecificnucleusgivesitssignal.Thesignalsareallmeasuredfromareferencezeropoint–theNMRsignalfromareferencesubstance.Thissubstancehastobechosensothatitssignalisstrongandsothatthereisonlyasinglepeakinthespectrumduetothissubstance–i.e.thenucleigivingthesignalareallinidenticalenvironments.Thesubstancealsohastobechemicallyunreactive,andvolatilesoitcanberemovedfromthesampleafterrunningthespectrum.Tetramethylsilane(TMS)(CH3)4Siisusedbecauseithasfouridenticalcarbonenvironments.WhenNMRspectraofasubstancearerecorded,asmallamountofTMSisaddedsothatthespectrometercanbecalibrated.ThesignalfromtheTMSistakenastheδ=0ppmchemicalshiftpointfromwhichthechemicalshiftsofotherpeaksaremeasured.

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CH

HH

C O HH

H

Proton(1H)NMR–signalsarisefromthehydrogenatomsinthemoleculeTheterms"proton"and"hydrogen"areusedinterchangeablyinNMR.Ahydrogen-1nucleusconsistsofoneproton.WhenreferringtoprotonNMRweareNOTtalkingabouttheprotonsinanyotheratoms'nuclei!ProtonNMRissimilarto13CNMRinanumberofimportantways -eachequivalenthydrogenatomsenvironmentproducesapeakinthespectrum -thepeakspositionsaremeasuredinchemicalshift(δ)inppm -thereferenceisstillTMS,whichhas12equivalenthydrogenatomsTherearesomeimportantdifferencestoo -theareaunderapeaktellsushowmanyhydrogenatomsthereareintha environment -eachpeakalsogivesinformationabouthowmanyprotonsareinadjacent environments(spin-spincoupling,seelater)The1Hisotopeismuchmoreabundant(99%)thanthe13Cisotope,sotheNMRsignalsaremuchstronger(meaningthatmuchsmallersamplesizescanbeused).Thechemicalshiftsfromprotonsindifferentenvironmentsaremuchsmallerthanfoundin13CNMR,andthechemicalshiftrangesforeachsignaltendtooverlapmore.StartingtointerpretaprotonNMRspectrum:Here'stheprotonNMRspectrumofethanol.Wecanusethechemicalshifttabletoworkoutwhichpeakbelongstowhichhydrogenatomsinthemolecule:

• Thepeakat5.4ppmisoutsidetherangeofC-HandisgoingtobetheO-Hproton

• Thepeakat3.7ppmcorrespondstoprotonsonacarbonwithanOalsoconnected (O-CH2-R)

• Thepeakat1.2ppmcorrespondstoanR-CH3protonenvironment.

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Noticethatthepeakheightsfollowthesamepatternasthenumberofprotonscontributingtoeachpeak.Infactit’stheareaunderthepeakthattellsusaboutthenumbersofprotons.NMRspectraareoftensuppliedwiththeareasunderthepeakscalculatedandlabeled:Spin-SpinCouplingFewNMRspectrometersaresolowinresolutionthattheyproducethekindofprotonNMRspectrumyouhaveseensofar–higherresolutionspectrometersshowanextrakindofinformation–spin-spincoupling.Comparethepreviousethanolspectrumwiththatproducedbyamorenormalhigherresolutioninstrument:Noticethatthepeakat1.2ppmhasbeensplitintotheefinerpeaks–atriplet–andthatthepeakat3.7ppmhasbeensplitintofourfinerpeaks–aquadruplet.Thepatternofpeakheightsischaracteristictoo:thetriplethasheights1:2:1andthequadruplethasheights1:3:3:1.

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Spin-spincouplingarisesfromtheinteractionoftheprotonsgivingrisetothepeak,withnon-equivalentprotonsonadjacentcarbonatoms.Thesplittingpatterntellsyouhowmanyprotonstherearebondedtotheadjacentcarbonatom.Notethat–OH,-NH(and–SH)groupsdonotcausesplittingandarenotthemselvessplit.Weusean"n+1"rule:Iftherearennon-equivalentprotonsontheadjacentcarbonthenthesplittingwillbeinton+1finerlines. -aquadruplet/quartettellsusthereare3hydrogenatomsontheadjacentcarbon -atriplettellsusthereare2hydrogenatomsontheadjacentcarbon -adoublettellsusthereis1hydrogenatomontheadjacentcarbon -asinglettellsustherearenohydrogenatomsontheadjacentcarbon (ornoadjacentcarbon!)Morecomplicatedsplittingpatterns(multiplet)aresometimesseenwhenthereareverysimilarbutnotequivalentprotonenvironmentse.g.inasubstitutedbenzenering,andherewerelyonthepeakareaandchemicalshiftinformationratherthantryingtointerpretthemultiplet.Seehowthisworksfortheethanolspectrum:

! Forthepeakat1.2ppm:Ithasanareaof3soitarisesfromthreeequivalentprotonsinanR-CHenvironment,sowearelookataCH3group.Itissplitintoatripletbecausetherearetwonon-equivalenthydrogensontheadjacentcarbonatom,sowecanidentifythissignalascomingfrom:–CH2-CH3

! Forthepeakat3.7ppm:Ithasanareaof2soitarisesfromtwoequivalentprotonsinanO-CH

environment,sowearelookingatan-O-CH2-fragmentofthemolecule.Itissplitintoaquadrupletbecausetherearethreenon-equivalenthydrogensontheadjacentcarbon,whichthereforemustbeaCH3group,sowecanidentifythissignalascomingfrom–O-CH2-CH3

! Notethatthequadruplet/tripletpatternweseehereiscommonlyseen,andcharacteristicofanethylgroupinthemolecule(-CH2-CH3).

! Forthepeakat5.4ppm:Ithasanareaof1soitarisesfromasingleprotoninan–OH

environment.Itisasinglet(nosplitting)as–OHgroupsarenotsplitanddonotcausesplitting.Spin-spincouplingonlyoccursbetweennon-equivalentprotonsonadjacentcarbonatoms.Agoodillustrationofthisisgivenbythe1HNMRspectraofdichloroethaneisomers–thespectrumof1,1-dichloroethaneconsistsofaquadruplet(peakarea=1)andadoublet(peakarea3)correspondingtothe–CHCl2and-CH3carbonsrespectively;butthespectrumof1,2-dichloroethanecontainsasinglepeak,unsplit,nottwooverlaidtriplets.

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Workedexample:Whatcanyouworkoutaboutthisspectrum?

NUMBEROFPEAKS=NUMBEROFENVIRONMENTS Therearetheepeaks,sotherearethreedifferentprotonenvironmentsFOREACHPEAK: Chemicalshift=whatenvironment Peakarea=howmanyhydrogensinthatenvironment…specifyfragment Splitting=informationaboutadjacentenvironment…extendthefragmentPeakatδ=7.2ppm: Chemicalshiftindicatesprotonsonabenzenering. Areaindicates5ofthem,soC6H5-fragment. Splittingisamultiplet,thehydrogensonadjacentcarbons aroundtheringcausethissplitting,butitistoocomplexto analyseusingN+1rule.(Thisistypicalforsubstituted benzenerings).Peakatδ=2.6ppm: Chemicalshiftindicateshydrogensonacarbonnexttoa benzenering. Areaindicates2ofthem,so–CH2-C6H5. Quartetsplittingindicates3non-equivalenthydrogenson anadjacentcarbon.Thecarbonofthebenzeneringwhere this–CH2-groupconnectshasnohydrogensonit,sothese arethreenon-equivalentprotonsintheformofaCH3- groupbondedtothe–CH2-,asCH3-CH2-.Peakatδ=1.3ppm: ChemicalshiftindicateshydrogensinanR-CH environment. Areaof3indicatesthatthisisa–CH3group. Tripletsplittingindicatestwonon-equivalent hydrogensonanadjacentcarbon,so–CH2-CH3. COMBINEFRAGMENTSFORFINALMOLECULE:

multiplet

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MoreaboutsolventsforNMRSpectroscopyNMRisusuallycarriedoutinsolution,soasolventisneeded.OrganicsolventscontainCandHatoms,andthesewouldproduceNMRsignalsthatwouldcomplicatethespectra.Instead,itiscommontousedeuteratedsolvents.The1Hisotopeisonlyoneoftheisotopesofhydrogen.The2Hisotope(withoneprotonandoneneutroninitsnucleus)iscalleddeuteriumandhasthechemicalsymbolD.Becausetheyareisotopesofthesameatom,hydrogenanddeuteriumhavethesamechemicalproperties.DeuteriumdoesnotproduceanNMRsignalinanNMRspectrometertunedtoprotonfrequencies,sosolventswithdeuteriumatomswherethehydrogenatomswouldhavebeenwilldothesamejobofdissolvingthesample,butproducenopeakinthespectrum.SolventssuchasCDCl3arecommonlyusedforprotonand13CNMR(inthelattercaseitiseasytoremovetheonepeakfromtheCinthesolvent).BecauseCDCl3isvolatileitseasytoevaporateoffthesolventandrecoverthesampleaftertheNMRexperiment.RememberthatasmallamountofTMSisalsoadded;notasasolventbutasareference.NMRspectrafromcompoundswith–OHand–NHgroups (alcohols,amines,carboxylicacids,phenols)Thepeaksarisingfrom–OHand–NHprotonsareoftenbroad,andoccuroverawiderangeofchemicalshiftvalues.Thismakesthemhardtoidentifyunambiguously.Thereisaclevertechniquewecanusetocheckifapeakwhichwethinkarisesfroman–OHor–NHgroupactuallydoes.ThismakesuseofD2O–watermoleculeswithbothhydrogenatomsreplacedwithdeuteriumatoms(sometimesreferredtoas"heavywater"sinceitsMris20,not18).1. RunthenormalNMRspectrumandmakepeakassignmentsasbestyoucan2. AddasmallamountofD2Otothesampleandshake(becausethesamplesizesare

reallytiny,asmallamountofD2Ostillconstitutesanexcess)3. RuntheNMRspectrumagain–anypeakswhichcorrespondto–OHor–NHprotons

willdisappearfromthespectrumThisworksbecausethehydrogenatomson–OHand–NHarelabile–theyrapidlyexchangewiththedeuteriumatomsintheD2O–andofcoursetheDatomsdon'tgivea1HNMRsignalsothepeaksdisappear. e.g.CH3CH2OH+D2O⇌CH3CH2OD+HOD

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Splitting(orthelackofit)from–OHand–NHprotonsAsfarassplittingisconcerned,ignore–OHand–NHprotons.

! Theywillshowupasasingletwhichisnotsplitbyadjacentprotons! Theywillnotcausesplittingofthesignalfromadjacentprotons

Thepeakfrom–OHand–NHprotonsmaybequitebroad.Tracesofwaterinthesolvent,andhydrogenbondingbetweensamplemolecules,canformhydrogenbondswith–NHand–OHprotonswhichaffectstheenvironment(electroncloud)inthevicinityoftheseprotons.CombininganalyticalmethodsItismuchmorecommontohavetheresultsofseveraldifferenttypesofanalysis,ratherthanrelyingonasinglemethod.Commonlywemighthavetheresultsofelementalanalysis,infraredspectra,massspectraandNMRspectratoworkwith.Givensuchavarietyofdata,asystematicapproachtointerpretingitisneeded,andthefollowingsequenceworks:CONTEXT:usetheanyinformationgiveninthequestion,andtheresultsofelementalanalysis,tosuggestpossibilitiesandeliminateothers.Youmaybeabletodetermineempiricalformula,oridentify/rejectcertainfunctionalgroupsbeingpresent.INFRAREDSPECTROSCOPY:usetheinfraredspectrumtosuggestfunctionalgroupspresentinthemoleculeandtoeliminatethepossibilityofothersbeingpresent.MASSSPECTROMETRY–MOLECULARIONPEAK:identifythemolecularionpeak(athighestm/evalue,butbearinmindtheremaybeaverysmallm+1peakdueto1%ofcarbonatomsbeing13C)andusethism/evaluetosuggestMrforthemolecule.Iftheempiricalformulaisknown,themolecularformulacanbefound.Iftheempiricalformulaisnotknown,atyougetafeelforthesizeofthemolecule.Thenumberofcarbonatomscanbeestimated,takingintoaccountthemassofanyfunctionalgroupsyouhaveidentifiedasbeingpresent.N.Boccasionallyamoleculecanbeunstableenoughthateverymoleculefragmentswhenhitbytheelectronbeaminthemassspectrometer–therewillnotbeamolecularionpeak,butyouwon’tfigurethisoutuntiltherestofthedatasupportsacandidatemoleculewhichdoesnotfitwiththehighestm/epeakinthemassspectrum.NMRSPECTROSCOPY:workthroughtheNMRdataasexplainedalreadyinthistopic,identifyingthenumberofdifferentenvironments,andthenpeakbypeakbuildingupapictureofthefragmentsandhowtheyareconnected.Usethisdatatoproposeone(ormore)candidatemoleculesthatfitallthedata,includingcheckingthattheyfittheMrvalue.

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MASSSPECTROMETRY–FRAGMENTIONS:lookatthemostcommonfragmentsthatwouldarisefrombreakingbondsinthecandidatemolecules,workoutthemassofthesefragments,andlookforcorrespondingfragmentionpeaksinthemassspectrumtoconfirmthecandidatemoleculeortochoosebetweencandidates.