1、Organic ChemistryNuclear Magnetic ResonanceMolecular Spectroscopy a spectroscopic technique that gives us information about the number and types of atoms in a molecule,for example,about the number and types of hydrogen atoms using 1H-NMR spectroscopy carbon atoms using 13C-NMR spectroscopy phosphoru
2、s atoms using 31P-NMR spectroscopyNuclear Spin StatesuAn electron has a spin quantum number of 1/2 with allowed values of+1/2 and-1/2 this spinning charge creates an associated magnetic field in effect,an electron behaves as if it is a tiny bar magnet and has what is called a magnetic momentuThe sam
3、e effect holds for certain atomic nuclei any atomic nucleus that has an odd mass number,an odd atomic number,or both also has a spin and a resulting nuclear magnetic moment the allowed nuclear spin states are determined by the spin quantum number,I,of the nucleusNuclear Spin States a nucleus with sp
4、in quantum number has spin states;if I =1/2,there are two allowed spin states Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds1 1H H2 2H H1 12 2C C1 13 3C C1 14 4N N1 16 6O O3 31 1P P3 32 2S SElem entElem entN ucl
5、ear spinN uclear spinquantum quantum num ber(num ber(I I)N um ber ofN um ber ofspin statesspin states1/21/21 10 00 00 01/21/21 12 23 31 12 23 31 11/21/22 21 1Nuclear Spins in B0 within a collection of 1H and 13C atoms,nuclear spins are completely random in orientation when placed in a strong externa
6、l magnetic field of strength B0,however,interaction between nuclear spins and the applied magnetic field is quantized,with the result that only certain orientations of nuclear magnetic moments are allowed Nuclear Spins in B0 for 1H and 13C,only two orientations are allowedNuclear Spins in B0uIn an a
7、pplied field strength of 7.05T,which is readily available with present-day superconducting electromagnets,the difference in energy between nuclear spin states for 1H is approximately 0.120 J(0.0286 cal)/mol,which corresponds to electromagnetic radiation of 300 MHz(300,000,000 Hz)13C is approximately
8、 0.030 J(0.00715 cal)/mol,which corresponds to electromagnetic radiation of 75MHz(75,000,000 Hz)Nuclear Spin in B0 the energy difference between allowed spin states increases linearly with applied field strength values shown here are for 1H nucleiNuclear Magnetic Resonance when nuclei with a spin qu
9、antum number of 1/2 are placed in an applied field,a small majority of nuclear spins are aligned with the applied field in the lower energy state the nucleus begins to precess and traces out a cone-shaped surface,in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it
10、 precesses in the earths gravitational field we express the rate of precession as a frequency in hertzNuclear Magnetic ResonanceuIf the precessing nucleus is irradiated with electromagnetic radiation of the same frequency as the rate of precession,the two frequencies couple,energy is absorbed,and th
11、e nuclear spin is flipped from spin state+1/2(with the applied field)to-1/2(against the applied field)Nuclear Magnetic Resonance Figure 13.3 the origin of nuclear magnetic“resonance Nuclear Magnetic Resonance in NMR spectroscopy,resonance is the absorption of electromagnetic radiation by a precessin
12、g nucleus and the resulting“flip”of its nuclear spin from a lower energy state to a higher energy stateuThe instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal a recording in an NMR spectrum of a nuclear magnetic resonanceNuclear Magne
13、tic Resonance if we were dealing with 1H nuclei isolated from all other atoms and electrons,any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H.The same is true of 13C nuclei but hydrogens in organic molecules are not isolated from all o
14、ther atoms;they are surrounded by electrons,which are caused to circulate by the presence of the applied field the circulation of electrons around a nucleus in an applied field is called and the nuclear shielding resulting from it is called Nuclear Magnetic Resonance the difference in resonance freq
15、uencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small the difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz,which is 1.2 parts per million(ppm)compared with the irradia
16、ting frequency 3 36 60 0 H H z z3 30 00 0 x x 1 10 06 6 H H z z1 1.2 2=1 1.2 2 p pp pm m1 10 06 6=Nuclear Magnetic Resonance signals are measured relative to the signal of the reference compound tetramethylsilane(TMS)for a 1H-NMR spectrum,signals are reported by their shift from the 12 H signal in T
17、MS for a 13C-NMR spectrum,signals are reported by their shift from the 4 C signal in TMS the shift in ppm of an NMR signal from the signal of TMST Te et tr ra am m e et th hy yl ls si il la an ne e (T TM M S S)C CH H3 3S Si i C CH H3 3C CH H3 3C CH H3 3NMR SpectrometerNMR SpectrometeruEssentials of
18、an NMR spectrometer are a powerful magnet,a radio-frequency generator,and a radio-frequency detectoruThe sample is dissolved in a solvent,most commonly CDCl3 or D2O,and placed in a sample tube which is then suspended in the magnetic field and set spinninguUsing a Fourier transform NMR(FT-NMR)spectro
19、meter,a spectrum can be recorded in about 2 secondsNMR Spectrumu1H-NMR spectrum of methyl acetate the shift of an NMR signal to the left on the chart paper the shift of an NMR signal to the right on the chart paperEquivalent Hydrogens have the same chemical environment a molecule with 1 set of equiv
20、alent hydrogens gives 1 NMR signalH H3 3C CC CC CC C H H3 3H H3 3C CC C H H3 3C C H H3 3C C C C H H3 3C C l lC CH H2 2C C H H2 2C C l lPropanonePropanone(Acetone)(Acetone)1,2-D ichloro-1,2-D ichloro-ethaneethaneC yclopentaneC yclopentane2,3-D im ethyl-2,3-D im ethyl-2-butene2-buteneO OEquivalent Hyd
21、rogens a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each setC CH H3 3C CH HC Cl lC Cl lC Cl lC CC CC CH H3 3H HH HO OC C y yc cl lo op pe en nt t-a an no on ne e(2 2 s si ig gn na al ls s)1 1,1 1-D D i ic ch hl lo or ro o-e et th ha an ne e(2 2 s si ig gn n
22、a al ls s)(Z Z)-1 1-C Ch hl lo or ro o-p pr ro op pe en ne e(3 3 s si ig gn na al ls s)C Cy yc cl lo oh he ex xe en ne e (3 3 s si ig gn na al ls s)Signal AreasuRelative areas of signals are proportional to the number of H giving rise to each signaluModern NMR spectrometers electronically integrate
23、and record the relative area of each signalR RC CH H2 2O O R R(C CH H3 3)4 4S Si iA A r rC CH H3 3R RC CH H3 3R RC CC CH HR RC CC CH H3 3R RO O H HR RC CH H2 2O O H HA A r rC CH H2 2R RO OO OR RC CH H2 2R RR R3 3C CH HR R2 2N NH HR RC CC CH H2 2R RR R2 2C C=C CR RC CH HR R2 2R R2 2C C=C CH HR RR RC
24、CH HO OR RC CO OH HO OR RC CH H2 2C Cl lR RC CH H2 2B Br rR RC CH H2 2I IR RC CH H2 2F FA A r rH HO OO OR R2 2C C=C CH H2 2R RC CO OC CH H3 3R RC CO OC CH H2 2R RA A r rO O H H9 9.5 5-1 10 0.1 13 3.7 7-3 3.9 93 3.4 4-3 3.6 6T Ty yp pe e o of f H H y yd dr ro og ge en n0 0 (b by y d de ef fi in ni it
25、 ti io on n)T Ty yp pe e o of f H H y yd dr ro og ge en nC Ch he em m i ic ca al l S Sh hi if ft t (Chemical Shift-1H-NMRChemical ShiftuDepends on(1)electronegativity of nearby atoms,(2)the hybridization of adjacent atoms,and(3)diamagnetic effects from adjacent pi bondsuElectronegativityC CH H3 3O O
26、 H HC CH H3 3F FC CH H3 3C Cl lC CH H3 3B Br rC CH H3 3I I(C CH H3 3)4 4C C(C CH H3 3)4 4S Si iC CH H3 3-X XE El le ec ct tr ro on ne eg g-a at t i iv vi it ty y o of f X XC Ch he em m i ic ca al l S Sh hi if ft t (Chemical ShiftuHybridization of adjacent atomsR RC CH H3 3,R R2 2C CH H2 2,R R3 3C CH
27、 HR R2 2C C=C CH HR R,R R2 2C C=C CH H2 2R RC CH HO OR R2 2C C=C C(R R)C CH HR R2 2R RC CC CH HA A l ll ly yl li ic cT Ty yp pe e o of f H H y yd dr ro og ge en n(R R =a al lk ky yl l)N N a am m e e o of fH H y yd dr ro og ge en nC C h he em m i ic ca al l S Sh hi if ft t (Chemical ShiftuDiamagnetic
28、 effects of pi bonds a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield(to the right)to a smaller value a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield(to the left)to a larger valueR RC CH H3 3R R2 2C C=C CH H2 2R RC CC
29、CH HT Ty yp pe e o of f H HN N a am m e eA A l lk ky yl lV V i in ny yl li ic cA A c ce et ty yl le en ni ic c0 0.8 8-1 1.0 04 4.6 6 -5 5.7 72 2.0 0 -3 3.0 0C Ch he em m i ic ca al l S Sh hi if ft t (Chemical Shift magnetic induction in the pi bonds of a carbon-carbon triple bond(Fig 13.9)Chemical S
30、hift magnetic induction in the pi bond of a carbon-carbon double bond(Fig 13.10)Chemical Shift magnetic induction of the pi electrons in an aromatic ring(Fig.13.11)Signal Splitting;the(+1)Rule the units into which an NMR signal is split;doublet,triplet,quartet,etc.splitting of an NMR signal into a s
31、et of peaks by the influence of neighboring nonequivalent hydrogens if a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s),its 1H-NMR signal is split into(n+1)peaksSignal Splitting(n+1)1H-NMR spectrum of 1,1-dichloroethaneC CH H3 3-C CH H-C
32、Cl lC Cl lF Fo or r t th he es se e h hy yd dr ro og ge en ns s,n n =1 1;t th he ei ir r s si ig gn na al l i is s s sp pl li it t i in nt to o(1 1 +1 1)=2 2 p pe ea ak ks s;a a d do ou ub bl le et tF Fo or r t th hi is s h hy yd dr ro og ge en n,n n =3 3;i it ts s s si i g gn na al l i is s s sp pl
33、 li it t i in nt to o(3 3 +1 1)=4 4 p pe ea ak ks s;a a q qu ua ar rt te et tSignal Splitting(n+1):predict the number of 1H-NMR signals and the splitting pattern of eachC CH H3 3C CC CH H2 2C CH H3 3O OC CH H3 3C CH H2 2C CC CH H2 2C CH H3 3O OC CH H3 3C CC CH H(C CH H3 3)2 2O O(a a)(b b)(c c)Origin
34、s of Signal Splitting an interaction in which the nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals the separation on an NMR spectrum(in hertz)between adjacent peaks in a multiplet;a quantitative measure of the influence of the spin-spin coupling with adja
35、cent nucleiOrigins of Signal SplittingOrigins of Signal Splitting because splitting patterns from spectra taken at 300 MHz and higher are often difficult to see,it is common to retrace certain signals in expanded form1H-NMR spectrum of 3-pentanone;scale expansion shows the triplet quartet pattern mo
36、re clearlyCoupling Constants the distance between peaks in a split signal,expressed in hertz the value is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled8-11 H z8-11 H z8-14 H z8-14 H z0-5 H z0-5 H z0-5 H z0-5 H z6-8 H z6-8 H z11-18 H z11-18 H z5-10 H z5-10 H z0-
37、5 H z0-5 H zC CC CH Ha aC CC CH Hb bH Ha aC CH Hb bC CH Ha aH Hb bH Ha aH Hb bH Ha aH Hb bH Hb bH Ha aH Hb bH Ha aC CC CH Ha aH Hb bOrigins of Signal SplittingSignal SplittinguPascals Triangle as illustrated by the highlighted entries,each entry is the sum of the values immediately above it to the l
38、eft and the rightPhysical Basis for(+1)RuleuCoupling of nuclear spins is mediated through intervening bonds H atoms with more than three bonds between them generally do not exhibit noticeable coupling for H atoms three bonds apart,the coupling is referred to as vicinal couplingCoupling Constants an
39、important factor in vicinal coupling is the angle between the C-H sigma bonds and whether or not it is fixed coupling is a maximum when is 0 and 180;it is a minimum when is 90More Complex Splitting Patterns thus far,we have concentrated on spin-spin coupling with only one other nonequivalent set of
40、H atoms more complex splittings arise when a set of H atoms couples to more than one set H atoms a tree diagram shows that when Hb is adjacent to nonequivalent Ha on one side and Hc on the other,the resulting coupling gives rise to a doublet of doublets More Complex Splitting Patterns if Hc is a set
41、 of two equivalent H,then the observed splitting is a doublet of tripletsMore Complex Splitting Patterns because the angle between C-H bond determines the extent of coupling,bond rotation is a key parameter in molecules with relatively free rotation about C-C sigma bonds,H atoms bonded to the same c
42、arbon in CH3 and CH2 groups generally are equivalent if there is restricted rotation,as in alkenes and cyclic structures,H atoms bonded to the same carbon may not be equivalent nonequivalent H on the same carbon will couple and cause signal splitting this type of coupling is called More Complex Spli
43、tting Patterns in ethyl propenoate,an unsymmetrical terminal alkene,the three vinylic hydrogens are nonequivalent More Complex Splitting Patterns a tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoateMore Complex Splitting Patterns cyclic structures often have res
44、tricted rotation about their C-C bonds and have constrained conformations as a result,two H atoms on a CH2 group can be nonequivalent,leading to complex splittingMore Complex Splitting Patterns a tree diagram for the complex coupling in 2-methyl-2-vinyloxiraneMore Complex Splitting PatternsuComplex
45、coupling in flexible molecules coupling in molecules with unrestricted bond rotation often gives only m+n+I peaks that is,the number of peaks for a signal is the number of adjacent hydrogens+1,no matter how many different sets of equivalent H atoms that represents the explanation is that bond rotati
46、on averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar;for example in the 6-to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms More Complex Splitting Patterns simplification of signal splitting occurs when coupling constants
47、 are the sameMore Complex Splitting Patterns an example of peak overlap occurs in the spectrum of 1-chloro-3-iodopropane the central CH2 has the possibility for 9 peaks(a triplet of triplets)but because Jab and Jbc are so similar,only 4+1=5 peaks are distinguishableStereochemistry&TopicityuDepending
48、 on the symmetry of a molecule,otherwise equivalent hydrogens may be homotopic enantiotopic diastereotopicuThe simplest way to visualize topicity is to substitute an atom or group by an isotope;is the resulting compound the same as its mirror image different from its mirror image are diastereomers p
49、ossibleStereochemistry&TopicityuHomotopic atoms or groups homotopic atoms or groups have identical chemical shifts under all conditionsAchiralAchiralHCHClClHCDClClDichloro-Dichloro-methanemethane(achiral)(achiral)Substitution does not Substitution does not produce a stereocenter;produce a stereocent
50、er;therefore hydrogenstherefore hydrogensare homotopic.are homotopic.Substitute Substitute one H by Done H by DAchiralAchiralHCHClClHCDClClDichloro-Dichloro-methanemethane(achiral)(achiral)Substitution does not Substitution does not produce a stereocenter;produce a stereocenter;therefore hydrogensth
