1、Chemical ShiftChemical Shift =B0 For a same kind of nucleus,the 0 should be the same!The magnetic field felt by the nucleus is not the same as applied fieldThe e-cloud that surrounds the nucleus has motion and generates a magnetic moment.This magnetic field alters B0 in the micro-environment around
2、the nucleus This electronic modulation of the B0 field is called magnetic shielding,B=B0(1-)Chemical ShiftMagnetic field at the nuclei is NOT equal to the applied field because of shielding or deshielding of electrons around the nuclei.Variation of the resonance frequency with shielding is called th
3、e chemical shiftB=B0(1-)=B0-Bloc=B0(1-)HO-CH2-CH3 olowfieldhighfield ExampleethanolChemical Shift Scale=B0 =B0(1-)/2=B0(1-2)/21）B0 is constant=(S-R)/R=(S-R)/(1-R)=(S-R)(R1)2)0 is constant=(BS-BR)/BR=(S-R)/(1-R)=(S-R)(R1)=(S-R)ppm (10-6)Standard ReferencesH3CSiCH3CH3CH3 =0The most common reference co
4、mpound is tetramethylsilane,TMSTMS is used as reference because it is soluble in most organic solvents,is inert,volatile,and has 12 equivalent 1Hs and 4 equivalent 13Cs.TMS is not water soluble,so cant be used in aqueous media,then use 2,2-dimethyl-2-silapentane-5-sulfonic acid,DSS(=0.015 ppm)Other
5、references can be used,such as the residual solvent peak,dioxane for 13C.ExampleLots of electrons nearbyFew electrons nearbyChemical shifts are influenced by the electronic environment.Therefore,they are diagnostic for particular types of molecular structures.The following figure indicates average r
6、anges of proton chemical shifts for different types of molecules.Different functional groups experience different effective fieldScales for Different Nuclei.The shielding of different nuclei depends on the electron density in its surroundings.dia(diamagnetic):arises from the magnetic field opposing
7、Bo from the electrons immediately surrounding the nucleus(s orbitals).para(paramagnetic):generated by electrons in p orbitals(as well as bonds).It is in favor of Bo.l:which is due to neighboring groups,and it can add or subtract from B0,depending on the nature of the group and its spatial orientatio
8、n.l is the term that actually makes nuclei with similar characteristics in a molecule have different shieldings(and therefore chemical shifts).Origins of (Bloc)=dia+para+lpara will dominate because we have more p electron shells that can become occupied(lower energy than p orbitals in 1H)1H 13C1H ha
9、s only a 1s orbital,dia will dominateOrigins of (Bloc)The shielding is determined by the electron density and for an isolated 1H,it is calculated with the Lamb formula:r r(r)is very complicated:We have to consider s,p,d,etc.,atomic orbitals,and we also have to consider molecular orbitals.This is the
10、 realm of quantum mechanical chemical shift calculations,far more than what we want to know.However,most of the effects can be qualitatively described by using dia and 1 for protons(remember that para has little contributions in 1H).We will start with effects(or contributions)to 1.These are known as
11、 inductive and conjugative effects,and as we will see,their origin lies in how the electron density around the 1H is affected by different species attached to it.200()3eerr drmrInductive Effects Since we lower the electron density around the nucleus,it will become deshielded,and therefore it will mo
12、ve towards lower fields(higher chemical shift).For example,if we consider hydrogen halides,well see that the more electronegative the halide is,the drop in shielding constants is inversely proportional to the E of the halide:(HF)(HCl)(HBr)(HI)H(1s)H(1s)C(sp3)An isolated 1H atom：Lamb formula 17.8 ppm
13、 for dia:Now,we add a-CH3 to it(and get methane),the electron cloud on the 1H(on any of the 4)will become deformed,because the electronegativity(E)of the carbon will pull the 1s electron of the 1H towards it:Inductive Effects The inductive effect on the shielding of the 1H is not limited to groups b
14、onded directly to it.We have to remember that the electron density around the 1H depends on the molecular orbitals of the whole molecule.The effects of electronegativity are transmitted through molecular orbitals(bonds):If we have a very electronegative atom bond to a carbon,protons bonded to that c
15、arbon will have their 1s electrons pulled away more than if we did not have the electronegative group.So,for the methane series we have:H-CH3H-CH2IH-CH2Br H-CH2ClH-CH2F2.12.52.83.04.00.231.982.452.844.13E Furthermore,we dont need a particularly electronegative atom.If we lengthen the carbon chain,th
16、e deshielding will also increase.Conjugative Effects Now we have an olefinic or aromatic proton and we have a substituent that can have different conjugative effects(+M or M).1)Ethene and EWGs or EDGs as substituents.If we consider methylvinylketone,the chemical shifts of the olefinic protons will m
17、ove downfield considerably.Because the ketone(an EWG)is taking electrons away from the double bond,the electron density around the 1H will diminish:If on the other hand we consider methylvinylether,the situation is reversed:The methoxy groups(an EDG),will donate electrons to the double bond,which wi
18、ll increase the electron density around the 1H and therefore increase the shielding:5.255.25OHHH6.356.376.12OHHH4.184.04EWG/EDG=Electron Withdrawing/Donating Group 2)1Hs on substituted aromatic systems.For example,in aniline we have an EDG,which has a+M effect.Since well have more electron density i
19、n the ring,all protons will be more shielded than the respective protons in benzene(7.24 ppm).Furthermore,if we draw resonant structures we can see that the ortho and para positions will have a larger electron density.Therefore,protons attached to the ortho or para carbons will be more shielded(lowe
20、r chemical shift):Conjugative Effects 7.24 On the other hand,nitrobenzene,which has an EWG,has a-M effect.All centers will have a lower electron density,but the ortho and para positions will have a particularly lowered electron density.All protons in nitrobenzene will be more deshielded than benzene
21、.In particular,the effect at the ortho and para positions will be the largest.Conjugative Effects Anisotropic Effects CH3-CH3 CH2=CH2 CHCH 0.88 5.31 1.48The chemical shifts of compounds with double and triple bonds and those of aromatic systems cannot be solely explained on the basis of shielding an
22、d deshielding as in the case of aliphatic compounds(the local diamagentic effects).Compounds with double and triple bonds in addition to diamagnetic shielding exhibit magnetic anisotropy due to delocalized electrons.Magnetic AnisotropyAnisotropic EffectsNow,we have a molecule like H-Y and Y can be a
23、n atom or group(ie.CCH).When we add a field B0,the electrons around Y will get movement which creats magnetic moment:Y=B0 Y B=B0(2-)/3r3 =-(2-)/3r3 B=(-)B0 Anisotropic EffectsC-HC-CC=CC=ON=OCC-90140150280370-340-901401504201300-340-1.5-22-24-55-130547.5111211-44-277.5111244175-27Ring Current Effects
24、 One of the most pronounced effects arising from induced magnetic moments in a chemical group are due to aromatic rings.If we consider the ring current of the ring,it will generate a magnetic field perpendicular to the plane of the ring,that will be against the external magnetic field:As we see,the
25、field lines through the ring are against of the external magnetic field(the induced magnetic moment will oppose the effect of Bo),but the return lines,which go on the outside of the ring,are in favor of it.Therefore,we can safely assume that protons sitting on the plane of the ring and thereabouts w
26、ill be deshielded,while those lying on top or below the ring will be shielded(i.e.,higher fields and therefore lower chemical shifts).e-BoBringRing Current EffectsExample:Deshielding in AlkenesDownfield(deshielded)shiftof vinyl hydrogensRing Current EffectsExample:Shielding in Alkenessp hydrogens ar
27、e shielded(upfield shift)Ring Current EffectsExample:Shielding/Deshielding in Aromatic CompoundsDownfield(deshielded)shiftof aromatic hydrogensProtons residing above and below the plane of aromatics are shielded(upfield shift)There are several formulas with different degrees of precision,but even th
28、e simplest ones give us a pretty decent estimate.The simplest one is the Pople point-dipole model:Ring Current EffectsAs was the case for single,double,and triple bonds,we can plot the value of the shielding as a function of the position in space of the 1H under study.It will also be cone-shaped,with a shielding regions(+,lower chemical shift),and deshielding regions(-,higher chemical shift).Protons on the sides of the aromatic ring will feel a higher local magnetic field(higher ppms),while those on top or bottom will feel a lower local magnetic field(lower ppms).谢谢观赏
