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OCR Chemistry A H432 Nuclear Magnetic Resonance Page 1 Nuclear Magnetic Resonance Spectroscopy (NMR) What is it ? An instrumental method that gives very detailed structural information about molecules. It can tell us - how many of certain types of atom a molecule contains - where these atoms are located in the molecule It works because the nuclei of certain atoms have a property we call "spin". Their spins can be "up ()" or "down ()". Normally these two spins have the same energy, but when the nuclei are in a magnetic field, the spins align with the field. The spin state which is aligned with the field is of lower energy than the spin state which opposes the field, so there is an energy gap – nuclei can be excited, using a pulse of radio frequency energy, from the lower energy spin state to the higher energy spin state, and can relax back to the lower energy spin state by releasing energy. The energies involved are tiny, so NMR instrumentation has to be extremely sensitive. The commonly-used nuclei for NMR are 1 H, and 13 C although it is also possible to do NMR with 31 P and 19 F amongst other atoms. Carbon-13 NMR – signals arise from the carbon atoms in the molecule 1.1% of all naturally occurring carbon atoms are 13 C atoms. They occur randomly in any position in the molecule where a carbon atom exists, so the signal from the 13 C atoms gives information about all the C atoms present in the sample. If each carbon nuclei was entirely on its own, separated from the influence of any other nuclei, it would produce an NMR signal at exactly the same energy. In reality the carbon atoms have other atoms bonded around them. The electrons in these atoms subtly affect the magnetic field the nucleus experiences, and therefore the signal occurs at a different energy. The energy of an NMR signal is measured in terms of how far away it is (the Chemical Shift, δ) from a known reference signal, in units of parts per million (ppm). Consider a molecule of propan-2-ol: Each carbon atom produces an NMR signal, but how many different carbon-atom environments are there ? The first and third carbons are in identical environments, so produce signals at the same chemical shift. The second carbon will produce a signal at a different chemical shift. We will therefore see two peaks in the NMR spectrum. H C H C H C H H H H O H

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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.