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,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,#,Spin-spin Coupling,dipole-dipole interaction：,magnetic dipole interaction between,nuclei,directly,Spin-spin Coupling,：,It will occur between two magnetically active nuclei that are connected through chemical bonds.We can see it for two atoms directly connected,or for atoms that see one another across several bonds.,Couplings are perhaps the most important parameter in NMR,as they allow us to elucidate chemical structure.,I,S,J,0,I,S,J=0,Spin-spin Couplingdipole-dipol,1,Example,E,thanol:,CH,3,CH,2,OH,T,hree singlets,The spectrum is a result of,spin-spin coupling,(,scale coupling,),ExampleEthanol:CH3CH2OH The,2,J,Coupling,E,ach nucleus act as a small magnet and will affect the magnetic moment of the other nuclei nearby(The energy levels of a nucleus will be affected by the spin state of nuclei nearby).The two nuclei that show this are said to be,coupled,to each other.,The two nuclei,couple,leading to,splitting,of the two lines associated with each nucleus.Note that the interaction is mutual,and the same splitting is observed for both resonances.,A splitting of a signal means that we have more energies involved in the transition of a certain nuclei.,The strength of the coupling is called the,coupling constant,J,separation between the multiplet lines.,I,A,and,I,X,are the nuclear spin vectors,which,are proportional to,m,A,and,m,B,the magnetic moments of the two nuclei.,J,A,X,is the,scalar coupling constant,.,It does not matter if we have a 60,a 400,or an 800 MHz,magnet,.,T,he,coupling,constants,are always the same!,J CouplingEach nucleus act as,3,J,Coupling,The resonance frequency of H,a,depends,on whether the adjacent,H,b,has spin up,or down.,The frequency separation betweenthe,two peaks is called,J,(,scalar coupling,constant,),(,typically,0,15 Hz,),.,The coupling constant is,independent,of,B,o,and is reported in Hz.,J Coupling The resonance frequ,4,J,Coupling,The resonance frequency of H,a,depends,on whether the adjacent,H,b,has spin up,or down.,The frequency separation betweenthe,two peaks is called,J,(,scalar coupling,constant,),(,typically,0,15 Hz,),.,The coupling constant is,independent,of,B,o,and is reported in Hz.,J Coupling The resonance frequ,5,Example,E,thanol:,CH,3,CH,2,OH,Spin-spin coupling tells us how many nuclei there are near to a given nucleus.,ExampleEthanol:CH3CH2OHSpin,6,J,Coupling,J Coupling,7,J,Coupling,Pascal Triangle,1,1 1,1 2 1,1 3 3 1,1 4 6 4 1,1 5 10 10 5 1,J CouplingPascal Triangle,8,J,Coupling,If a proton interacts with two different sets of equivalent protons,the multiplicity will be:,spin system,A,n,M,m,X,p,(m+1)(p+1),Consider C,H,3,C,H,2,CH,2,I,H,=1.02;,H,=1.86;,H,=3.17,-C,H,3,;1:2:1 triplet(from methylene C,H,2,),-C,H,2,;quartet of triplets 12 lines(3+1)(2+1),-CH,2,;1:2:1 triplet(from methylene C,H,2,),Coupling between Sets of Spins,:,J CouplingIf a proton interact,9,J,Coupling,Factors affect the size of J:,The numbers of bonds involved,The dihedral angle,for vicinal protons,The configurations,The conformations,Karplus Equation,J couplings provide an estimation of molecular conformation!,J()=Acos,2,()+Bcos()+C,A,B,and,C,are empirically derived parameters.,J CouplingFactors affect the s,10,A,1,=E,4,-E,2,=,n,A,-,1,/,2,J,A,X,A,2,=E,3,-E,1,=,n,A,+,1,/,2,J,A,X,X,1,=E,2,-E,1,=,n,X,-,1,/,2,J,A,X,X,2,=E,4,-E,3,=,n,X,+,1,/,2,J,A,X,J,Coupling,When B,0,paralells Z,the H of the system is:,Analysis of spin-spin coupling:,H=-,A,I,Az,-,X,I,Xz,+J,AX,I,A,I,X,m=1,H=-,A,I,Az,-,B,I,Xz,+J,AX,I,Az,I,Xz,E,4,=,1,/,2,n,A,+,1,/,2,n,X,+,1,/,4,J,A,X,E,3,=,1,/,2,n,A,-,1,/,2,n,X,-,1,/,4,J,A,X,E,2,=-,1,/,2,n,A,+,1,/,2,n,X,-,1,/,4,J,A,X,E,1,=-,1,/,2,n,A,-,1,/,2,n,X,+,1,/,4,J,A,X,B核跃迁频率,A核跃迁频率,E,A,X,A,X,A,1,A,2,A,1,A,2,B,o,J=0,J 0,E,4,E,3,E,2,E,1,A1=E4-E2=nA -1/2 JAX,11,J,Coupling,E,A,X,A,X,A,1,A,2,A,1,A,2,B,o,J=0,J 0,J J,.,B,0,Dn,Dn,J,A very simple way to determine if we have an,AB,system is by looking at the roofing effect:coupled pairs will lean towards each other,making a little roof:,ROOF,As with an,AX,system,J,AB,is the separation between lines,1,and,2,or,3,and,4,:,|,J,AB,|=|f,1,-f,2,|=|f,3,-f,4,|,Now,the chemical shifts of nuclei A and B are not at the center of the,doublets.They,ll be at the center of mass of both lines.This can be calculated as follows.Being,Dn,the,n,A,-,n,B,chemical shift difference:,Dn,2,=|(f,1,-f,4,)(f,2,-f,3,)|,n,A,=,n,Z,-,Dn,/2,n,B,=,n,Z,+,Dn,/2,I,1,I,2,I,3,I,4,I,2,I,3,|f,1,-f,4,|,=,I,1,I,4,|f,2,-f,3,|,Analysis of 2nd Order Systems,16,Magnetic&Chemical Equivalence,As we saw last time,we can do a very simple first order analysis of this spin system,because we assumed that all CH,2,protons were equal,and all CH,3,protons were equal.,We can easily see that they are chemically equivalent.Additionally,we have free rotation around the bond,which makes their chemical shifts and couplings equal.,S,o in effect,the coupling of any proton in CH,2,to any proton in the CH,3,will,be the same.,M,agnetic,equivalence,:,A,n ethoxy group(-O-CH2-CH3),Chemi
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