J耦合谱分析以及偶极耦合.pptx

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dipole-dipoleinteraction:
magneticdipoleinteractionbetweennucleidirectly
Spin-spinCoupling:
,orforatomsthat‘see’oneanotheracrossseveralbonds.
CouplingsareperhapsthemostimportantparameterinNMR,astheyallowustoelucidatechemicalstructure.
I S
J0
I S
J=0
Example
Ethanol:CH3CH2OH
Threesinglets
Thespectrumisaresultofspin-spincoupling(scalecoupling)
JCoupling
Eachnucleusactasasmallmagnetandwillaffectthemagneticmomentoftheothernucleinearby(Theenergylevelsofanucleuswillbeaffectedbythespinstateofnucleinearby).Thetwonucleithatshowthisaresaidtobecoupledtoeachother.
,andthesamesplittingisobservedforbothresonances.
Asplittingofasignalmeansthatwehavemoreenergiesinvolvedinthetransitionofacertainnuclei.
Thestrengthofthecouplingiscalledthecouplingconstant,J–separationbetweenthemultipletlines.
IAandIXarethenuclearspinvectorswhichareproportionaltomAand
mB,,a400,oran800MHz
!
JCoupling
TheresonancefrequencyofHadepends
onwhethertheadjacentHbhasspinup
ordown.
Thefrequencyseparationbetweenthe
twopeaksiscalledJ(scalarcoupling
constant)(typically0~15Hz).
Thecouplingconstantisindependentof
BoandisreportedinHz.
Example
Ethanol:CH3CH2OH
Spin-spincouplingtellsushowmanynucleithereareneartoagivennucleus.
JCoupling
JCoupling
PascalTriangle
1
11
121
1331
14641
15101051
JCoupling
Ifaprotoninteractswithtwodifferentsetsofequivalentprotons,themultiplicitywillbe:
—spinsystem,AnMmXp(m+1)(p+1)
ConsiderCH3CH2CH2I
—H=;H=;H=
-CH3;1:2:1triplet(frommethyleneCH2)
-CH2;quartetoftriplets–12lines(3+1)(2+1)
-CH2;1:2:1triplet(frommethyleneCH2)
CouplingbetweenSetsofSpins:
JCoupling
FactorsaffectthesizeofJ:
Thenumbersofbondsinvolved
Thedihedralangleφ,forvicinalprotons
Theconfigurations
Theconformations
KarplusEquation
Jcouplingsprovideanestimationofmolecularconformation!
J(θ)=Acos2(θ)+Bcos(θ)+C
A,B,andCareempiricallyderivedparameters.
A1=E4-E2=nA-1/2JAX
A2=E3-E1=nA+1/2JAX
X1=E2-E1=nX-1/2JAX
X2=E4-E3=nX+1/2JAX
JCoupling
WhenB0paralellsZ,theHofthesystemis:
Analysisofspin-spincoupling:
H=-νAIAz-νXIXz+JAXIA▪IX
△m=±1
H=-νAIAz-νBIXz+JAXIAzIXz
E4=1/2nA+1/2nX+1/4JAX
E3=1/2nA-1/2nX-1/4JAX
E2=-1/2nA+1/2nX-1/4JAX
E1=-1/2nA-1/2nX+1/4JAX
B核跃迁频率
A核跃迁频率
E
AX
AX
A1
A2
A1
A2
Bo
J=0
J>0
E4
E3
E2
E1