J耦合、譜分析以及偶極耦合課件

JcouplingSpin-spinCouplingdipole-dipoleinteraction：

magneticdipoleinteractionbetweennucleidirectlySpin-spinCoupling：

Itwilloccurbetweentwomagneticallyactivenucleithatareconnectedthroughchemicalbonds.Wecanseeitfortwoatomsdirectlyconnected,orforatomsthat‘see’oneanotheracrossseveralbonds.

CouplingsareperhapsthemostimportantparameterinNMR,astheyallowustoelucidatechemicalstructure.I

SJ0I

SJ=0ExampleEthanol:

CH3CH2OHThreesinglets

Thespectrumisaresultofspin-spincoupling(scalecoupling)JCoupling

TheresonancefrequencyofHa

depends

onwhethertheadjacent

Hb

hasspinup

ordown.

Thefrequencyseparationbetweenthe

twopeaksiscalledJ(scalarcoupling

constant)

(typically

0～15Hz).

Thecouplingconstantisindependentof

BoandisreportedinHz.JCoupling

TheresonancefrequencyofHa

depends

onwhethertheadjacent

Hb

hasspinup

ordown.

Thefrequencyseparationbetweenthe

twopeaksiscalledJ(scalarcoupling

constant)

(typically

0～15Hz).

Thecouplingconstantisindependentof

BoandisreportedinHz.ExampleEthanol:

CH3CH2OHSpin-spincouplingtellsushowmanynucleithereareneartoagivennucleus.JCouplingPascalTriangle11112113311464115101051JCouplingIfaprotoninteractswithtwodifferentsetsofequivalentprotons,themultiplicitywillbe: —spinsystem,AnMmXp

(m+1)(p+1)ConsiderCH3CH2CH2I

—H=1.02;H=1.86;H=3.17-CH3;1:2:1triplet(frommethyleneCH2)-CH2;quartetoftriplets–12lines(3+1)(2+1)-CH2;1:2:1triplet(frommethyleneCH2)CouplingbetweenSetsofSpins:JCouplingFactorsaffectthesizeofJ:ThenumbersofbondsinvolvedThedihedralangleφ,forvicinalprotonsTheconfigurationsTheconformationsKarplusEquationJcouplingsprovideanestimationofmolecularconformation!J(θ)=Acos2(θ)+Bcos(θ)+CA,B,andCareempiricallyderivedparameters.JCouplingEAXAXA1A2A1A2BoJ=0J<0E4E3E2E1A1A2n(A1)=n(A2)A2A1n(A2)n(A1)A1A2n(A1)n(A2)J=0J>0J<0Analysisofspin-spincoupling:Analysisof1stOrderSystemsCH3CH2CH3CH2J(Hz)4.5ppm1.5ppmEthylacetate(CH3COOCH2CH3):Nuclei

A

iscoupledto

n

identicalnucleiX(ofspin1/2),

A

willshowupas

n+1

linesinthespectrum.Sincewehavethesameprobabilityoffindingthesysteminanyofthestates,andstatesinthesamerowshaveequalenergy,theintensitywillhavearatio

1:2:1

fortheCH3,andaratioof

1:3:3:1

fortheCH2.PascalTriangle11112113311464115101051Inaspinsysteminwhichwehaveacertainnucleicoupledtomorethanonenuclei,allfirstorder,thesplittingwillbebasicallyanextensionofwhatwesawbefore.SaythatwehaveaCH(A)coupledtoaCH3(M)witha

JAM

of7Hz,andtoaCH2(X)witha

JAX

of3Hz.Webasicallygoinsteps.Firstthebigcoupling,whichwillgiveaquartet:Thenthesmallcoupling,whichwillspliteachlineinthequartetintoatriplet:Thisiscalleda

tripletofquartets

(bigeffectisthelast…).Analysisof1stOrderSystems7Hz)3Hz

Analysisof2ndOrderSystems1234ABnZnAnBWhatwehavebeendescribingsofarisaspinsysteminwhich

Dn>>J.B0

Dn

Dn

~

J

Averysimplewaytodetermineifwehavean

AB

systemisbylookingattheroofingeffect:coupledpairswillleantowardseachother,makingalittleroof:ROOFAswithan

AX

system,

JAB

istheseparationbetweenlines

1and2or

3and4:|JAB|=|f1-f2|=|f3-f4|Now,thechemicalshiftsofnucleiAandBarenotatthecenterofthe

doublets.They'llbeatthecenterofmassofbothlines.Thiscanbecalculatedasfollows.Being

Dn

thenA

-nB

chemicalshiftdifference:Dn2=|(f1-f4)(f2-f3)|nA

=nZ-Dn/2

nB

=nZ+Dn/2I1I2I3I4I2I3|f1-f4|==I1I4|f2-f3|Magnetic&ChemicalEquivalenceAswesawlasttime,wecandoaverysimplefirstorderanalysisofthisspinsystem,becauseweassumedthatallCH2protonswere‘equal’,andallCH3protonswere‘equal’.Wecaneasilyseethattheyarechemicallyequivalent.Additionally,wehavefreerotationaroundthebond,whichmakestheirchemicalshiftsandcouplingsequal.Soineffect,thecouplingofanyprotoninCH2toanyprotonintheCH3will

bethesame.Magnetic

equivalence:Anethoxygroup(-O-CH2-CH3)

ChemicalEquivalence:Thenucleigroupswhosechemicalshiftsareequalarechemicalequivalence.

d(CH2)>>d(CH3)

A2X3system

Wehave2magneticallyequivalent1HsontheCH2,and3ontheCH3.The

2JHH

coupling

iszerointhiscase,becausetheenergiesforanyofthe

three

protonsisthesame.Weuse

A

torefertotheCH2protons,and

X

torefertotheCH3protonsbecausethey

haveverydifferentds.Westartwiththeletter

Aforthemostdeshieldedspin.Magnetic&ChemicalEquivalenceCH2F2isanotherexampleofan‘AX’typesystem.Inthiscase,the1Hsandthe19Fsare

equalnotduetorotation,buttosymmetryaroundthecarbon.It’san

A2X2

system.

ForCH2F2,wecanalsocomparethecouplingstocheck

the1Hsand19Fsareequivalent:

JH1F1=JH1F2=JH2F1=JH2F2.Allduetotheirsymmetry.Symmetry

noRotationdHa

=

dHb

dFa

=

dFbduetothegeometryofthiscompoundJHaFa

JHaFb

JHbFa

JHbFbTheenergylevelsfor

Ha

andHb

aredifferent(notdegenerateanymoreasinCH3),andwehave

JHaHb

≠0.For

Ha,wehave

JHaFa,JHaFb,and

JHaHb.For

Hb,wehave

JHbFa,JHbFb,and

JHbHa.CH2F2H2C=CF2A2X2AA'XX'1H-NMRData:3componentsChemicalshift-eachnonequivalenthydrogengivesauniquesignal.J-Scalecouplingsproducesplittingpatterncorrelatedwithm