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7512J.Org.Chem.1997,62,7512-7515

NMRChemicalShiftsofCommonLaboratorySolventsasTraceImpurities

HugoE.Gottlieb,*VadimKotlyar,and

AbrahamNudelman*

DepartmentofChemistry,Bar-IlanUniversity,

Ramat-Gan52900,Israel

ReceivedJune27,1997

InthecourseoftheroutineuseofNMRasanaidfororganicchemistry,aday-to-dayproblemistheidentifica-tionofsignalsderivingfromcommoncontaminants(water,solvents,stabilizers,oils)inless-than-analyti-cally-puresamples.Thisdatamaybeavailableintheliterature,butthetimeinvolvedinsearchingforitmaybeconsiderable.Anotherissueistheconcentrationdependenceofchemicalshifts(especially1H);resultsobtainedtwoorthreedecadesagousuallyrefertomuchmoreconcentratedsamples,andrunatlowermagneticfields,thantoday’spractice.

Wethereforedecidedtocollect1Hand13Cchemicalshiftsofwhatare,inourexperience,themostpopular“extrapeaks”inavarietyofcommonlyusedNMRsolvents,inthehopethatthiswillbeofassistancetothepracticingchemist.

ExperimentalSection

NMRspectraweretakeninaBrukerDPX-300instrument(300.1and75.5MHzfor1Hand13C,respectively).Unlessotherwiseindicated,allwererunatroomtemperature(24(1°C).Fortheexperimentsinthelastsectionofthispaper,probetemperaturesweremeasuredwithacalibratedEurotherm840/Tdigitalthermometer,connectedtoathermocouplewhichwasintroducedintoanNMRtubefilledwithmineraloiltoap-proximatelythesamelevelasatypicalsample.Ateachtemperature,theD2Osampleswerelefttoequilibrateforatleast10minbeforethedatawerecollected.

Inordertoavoidhavingtoobtainhundredsofspectra,wepreparedsevenstocksolutionscontainingapproximatelyequalamountsofseveralofourentries,choseninsuchawayastopreventintermolecularinteractionsandpossibleambiguitiesinassignment.Solution1:acetone,tert-butylmethylether,di-methylformamide,ethanol,toluene.Solution2:benzene,di-methylsulfoxide,ethylacetate,methanol.Solution3:aceticacid,chloroform,diethylether,2-propanol,tetrahydrofuran.Solution4:acetonitrile,dichloromethane,dioxane,n-hexane,HMPA.Solution5:1,2-dichloroethane,ethylmethylketone,n-pentane,pyridine.Solution6:tert-butylalcohol,BHT,cyclo-hexane,1,2-dimethoxyethane,nitromethane,siliconegrease,triethylamine.Solution7:diglyme,dimethylacetamide,ethyl-eneglycol,“grease”(engineoil).ForD2O.Solution1:acetone,tert-butylmethylether,dimethylformamide,ethanol,2-propanol.Solution2:dimethylsulfoxide,ethylacetate,ethyleneglycol,methanol.Solution3:acetonitrile,diglyme,dioxane,HMPA,pyridine.Solution4:1,2-dimethoxyethane,dimethylacetamide,ethylmethylketone,triethylamine.Solution5:aceticacid,tert-butylalcohol,diethylether,tetrahydrofuran.InD2OandCD3ODnitromethanewasrunseparately,astheprotonsexchangedwithdeuteriuminpresenceoftriethylamine.

Results

ProtonSpectra(Table1).Asampleof0.6mLofthesolvent,containing1µLofTMS,1wasfirstrunonitsown.Fromthisspectrumwedeterminedthechemicalshiftsofthesolventresidualpeak2andthewaterpeak.Itshouldbenotedthatthelatterisquitetemperature-(1)ForrecommendationsonthepublicationofNMRdata,see:IUPACCommissiononMolecularStructureandSpectroscopy.PureAppl.Chem.1972,29,627;1976,45,217.

S0022-3263(97)01176-6CCC:$14.00Figure1.ChemicalshiftofHDOasafunctionoftempera-ture.

dependent(videinfra).Also,anypotentialhydrogen-bondacceptorwilltendtoshiftthewatersignaldown-field;thisisparticularlytruefornonpolarsolvents.Incontrast,ine.g.DMSOthewaterisalreadystronglyhydrogen-bondedtothesolvent,andsoluteshaveonlyanegligibleeffectonitschemicalshift.ThisisalsotrueforD2O;thechemicalshiftoftheresidualHDOisverytemperature-dependent(videinfra)but,maybecounter-intuitively,remarkablysolute(andpH)independent.Wethenadded3µLofoneofourstocksolutionstotheNMRtube.ThechemicalshiftswerereadandarepresentedinTable1.Exceptwhereindicated,thecouplingconstants,andthereforethepeakshapes,areessentiallysolvent-independentandarepresentedonlyonce.

ForD2Oasasolvent,theacceptedreferencepeak(δ)0)isthemethylsignalofthesodiumsaltof3-(trimeth-ylsilyl)propanesulfonicacid;onecrystalofthiswasaddedtoeachNMRtube.Thismaterialhasseveraldisadvan-tages,however:itisnotvolatile,soitcannotbereadilyeliminatedifthesamplehastoberecovered.Inaddition,unlessonepurchasesitintherelativelyexpensivedeuteratedform,itaddsthreemoresignalstothespectrum(methylenes1,2,and3appearat2.91,1.76,and0.63ppm,respectively).Wesuggestthatthere-sidualHDOpeakbeusedasasecondaryreference;wefindthatiftheeffectsoftemperaturearetakenintoaccount(videinfra),thisisveryreproducible.ForD2O,weusedadifferentsetofstocksolutions,sincemanyofthelesspolarsubstratesarenotsignificantlywater-soluble(seeTable1).Wealsoransodiumacetateandsodiumformate(chemicalshifts:1.90and8.44ppm,respectively).

CarbonSpectra(Table2).Toeachtube,50µLofthestocksolutionand3µLofTMS1wereadded.Thesolventchemicalshifts3wereobtainedfromthespectracontainingthesolutes,andtherangesofchemicalshifts

(2)I.e.,thesignaloftheprotonfortheisotopomerwithonelessdeuteriumthantheperdeuteratedmaterial,e.g.,CHCl3inCDCl3orC6D5HinC6D6.ExceptforCHCl3,thesplittingduetoJHDistypicallyobserved(toagoodapproximation,itis1/6.5ofthevalueofthecorrespondingJHH).ForCHD2groups(deuteratedacetone,DMSO,acetonitrile),thissignalisa1:2:3:2:1quintetwithasplittingofca.2Hz.

(3)Incontrasttowhatwassaidinnote2,inthe13Cspectrathesolventsignalisduetotheperdeuteratedisotopomer,andtheone-bondcouplingstodeuteriumarealwaysobservable(ca.20-30Hz).

©1997AmericanChemicalSociety