有机化学英文chapter13PPT课件

1、Nuclear Magnetic ResonanceNoImageNoImageNoImageNoImage第1页/共74页Molecular 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 s

2、pectroscopy phosphorus atoms using 31P-NMR spectroscopy第2页/共74页Nuclear Spin States An 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

3、called a magnetic moment The same 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 nucleus第

4、3页/共74页Nuclear Spin States a nucleus with spin 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

5、 16 6O O3 31 1P P3 32 2S SElementElementNuclear spinNuclear spinquantum quantum number (number (I I ) )Number ofNumber ofspin statesspin states1/21/21 10 00 00 01/21/21 12 23 31 12 23 31 11/21/22 21 1第4页/共74页Nuclear Spins in B0 within a collection of 1H and 13C atoms, nuclear spins are completely ra

6、ndom in orientation when placed in a strong external 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 第5页/共74页Nuclear Spins in B0 for 1H and 13

7、C, only two orientations are allowedNoImageNoImage第6页/共74页Nuclear Spins in B0 In an applied 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, whic

8、h corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz)13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz)第7页/共74页Nuclear Spin in B0 the energy difference between allowed spin states increases linearly with applied fi

9、eld strength values shown here are for 1H nuclei第8页/共74页Nuclear Magnetic Resonance when nuclei with a spin quantum 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

10、 a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it precesses in the earths gravitational field we express the rate of precession as a frequency in hertz第9页/共74页Nuclear Magnetic Resonance If the precessing nucleus is irradiated with electro

11、magnetic radiation of the same frequency as the rate of precession, the two frequencies couple, energy is absorbed, and the nuclear spin is flipped from spin state +1/2 (with the applied field) to -1/2 (against the applied field)第10页/共74页Nuclear Magnetic Resonance Figure 13.3 the origin of nuclear m

12、agnetic “resonance 第11页/共74页Nuclear Magnetic Resonance in NMR spectroscopy, resonance is the absorption of electromagnetic radiation by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state The instrument used to detect this coupling of

13、precession frequency and electromagnetic radiation records it as a signal a recording in an NMR spectrum of a nuclear magnetic resonance第12页/共74页Nuclear Magnetic Resonance if we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation th

14、at 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 other atoms; they are surrounded by electrons, which are caused to circulate by the presence of the applied field the circulation of electron

15、s around a nucleus in an applied field is called and the nuclear shielding resulting from it is called 第13页/共74页Nuclear Magnetic Resonance the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small the difference i

16、n 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 irradiating 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= =第14页/共74页Nuclear

17、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 TMS for a 13C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS the shift in

18、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 3第15页/共74页NMR Spectrometer第16页/共74页NMR Spectrometer Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a ra

19、dio-frequency detector The 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 spinning Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds第17页/共74页NMR Spectrum1

20、H-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 paper第18页/共74页Equivalent Hydrogens have the same chemical environment a molecule with 1 set of equivalent hydrogens gives 1 NMR signalH H3 3C CC CC CC C H H

21、3 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-Dichloro-1,2-Dichloro-ethaneethaneCyclopentaneCyclopentane2,3-Dimethyl-2,3-Dimethyl-2-butene2-buteneO O第19页/共74页Equivalent Hydrogens a molecule with 2 or more sets of equivalent hyd

22、rogens 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 na al ls s) ) ( (Z Z) )- -1 1- -C Ch hl l

23、o 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) )第20页/共74页Signal Areas Relative areas of signals are proportional to the number of H giving rise to each signal Modern NMR spectrometers electronically integrate and record

24、 the relative area of each signal第21页/共74页R 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

25、 RR RC 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

26、 n0 0 ( (b by y d de ef fi in ni it 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 ( ( ) )1 1. . 6 6- -2 2. . 6 62 2. . 0 0- -3 3. . 0 00 0. . 8 8- -1 1. . 0 01 1. . 2 2- -1 1. . 4 41 1. . 4 4- -1 1. . 7 72 2. . 1 1- -2 2. . 3 30 0. . 5 5- -6 6. .

27、 0 02 2. . 2 2- -2 2. . 6 63 3. . 4 4- -4 4. . 0 0C Ch he em m i ic ca al l S Sh hi if ft t ( ( ) )3 3. . 3 3- -4 4. . 0 02 2. . 2 2- -2 2. . 5 52 2. . 3 3- -2 2. . 8 80 0. . 5 5- -5 5. . 0 04 4. . 6 6- -5 5. . 0 05 5. . 0 0- -5 5. . 7 71 10 0- -1 13 34 4. . 1 1- -4 4. . 7 73 3. . 1 1- -3 3. . 3 33

28、3. . 6 6- -3 3. . 8 84 4. . 4 4- -4 4. . 5 56 6. . 5 5- -8 8. . 5 54 4. . 5 5- -4 4. . 7 7第22页/共74页Chemical Shift - 1H-NMR第23页/共74页Chemical Shift Depends on (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds Electronegat

29、ivityC CH H3 3O O 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 ( ( ) )4 4. . 0 03 3. . 5 53 3. . 1 12 2. . 8 82 2. . 5 5

30、2 2. . 1 11 1. . 8 84 4. . 2 26 63 3. . 4 47 73 3. . 0 05 52 2. . 6 68 82 2. . 1 16 60 0. . 8 86 60 0. . 0 00 0第24页/共74页Chemical Shift Hybridization of adjacent atomsR RC CH H3 3, , R R2 2C CH H2 2, , R R3 3C CH 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

31、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 ( ( ) )A A l lk ky yl lA A c ce et ty yl le en ni ic cV V i in ny yl li ic cA A l ld de eh hy yd

32、 di ic c0 0. . 8 8 - - 1 1. . 7 71 1. . 6 6 - - 2 2. . 6 64 4. . 6 6 - - 5 5. . 7 79 9. . 5 5- -1 10 0. . 1 12 2. . 0 0 - - 3 3. . 0 0第25页/共74页Chemical Shift Diamagnetic effects of pi bonds a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield (to the right) to a s

33、maller 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 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

34、 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 ( ( ) )第26页/共74页Chemical Shift magnetic induction in the pi bonds of a carbon-carbon triple bond (Fig 13.9)第27页/共74页Chemical Shift magnetic induction in the pi bond of a carbon-carbon double bond (Fi

35、g 13.10)第28页/共74页Chemical Shift magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11)第29页/共74页Signal Splitting; the (n + 1) Rule the units into which an NMR signal is split; doublet, triplet, quartet, etc. splitting of an NMR signal into a set of peaks by the influence of neighbori

36、ng 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) peaks第30页/共74页Signal Splitting (n + 1)1H-NMR spectrum of 1,1-dichloroethaneC CH H3 3- -C CH H- -C Cl lC Cl lF Fo or r

37、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

38、 s s sp pl 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 t第31页/共74页Signal 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(

39、 (C CH H3 3) )2 2O O( (a a) )( (b b) )( (c c) )第32页/共74页Origins 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 quantita

40、tive measure of the influence of the spin-spin coupling with adjacent nuclei第33页/共74页Origins of Signal Splitting第34页/共74页Origins 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 for

41、m1H-NMR spectrum of 3-pentanone; scale expansion shows the triplet quartet pattern more clearly第35页/共74页Coupling 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 Hz8-11 Hz8

42、-14 Hz8-14 Hz0-5 Hz0-5 Hz0-5 Hz0-5 Hz6-8 Hz6-8 Hz11-18 Hz11-18 Hz5-10 Hz5-10 Hz0-5 Hz0-5 HzC 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 b第36页/共74页Origins of Signal Splitting第37页/共74页Signal Splitting Pascals Triangle as illustr

43、ated by the highlighted entries, each entry is the sum of the values immediately above it to the left and the right第38页/共74页Physical Basis for (n + 1) Rule Coupling of nuclear spins is mediated through intervening bonds H atoms with more than three bonds between them generally do not exhibit noticea

44、ble coupling for H atoms three bonds apart, the coupling is referred to as vicinal coupling第39页/共74页Coupling Constants an important factor in vicinal coupling is the angle a between the C-H sigma bonds and whether or not it is fixed coupling is a maximum when a is 0 and 180; it is a minimum when a i

45、s 90第40页/共74页More Complex Splitting Patterns thus far, we have concentrated on spin-spin coupling with only one other nonequivalent set of 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 H

46、a on one side and Hc on the other, the resulting coupling gives rise to a doublet of doublets 第41页/共74页More Complex Splitting Patterns if Hc is a set of two equivalent H, then the observed splitting is a doublet of triplets第42页/共74页More Complex Splitting Patterns because the angle between C-H bond d

47、etermines 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 carbon in CH3 and CH2 groups generally are equivalent if there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded t

48、o 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 第43页/共74页More Complex Splitting Patterns in ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent 第44页/共74页Mor

49、e Complex Splitting Patterns a tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate第45页/共74页More Complex Splitting Patterns cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations as a result, two H atoms on a CH2

50、group can be nonequivalent, leading to complex splitting第46页/共74页More Complex Splitting Patterns a tree diagram for the complex coupling in 2-methyl-2-vinyloxirane第47页/共74页More Complex Splitting Patterns Complex coupling in flexible molecules coupling in molecules with unrestricted bond rotation oft

51、en 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 rotation averages the coupling constants throughout molecules with freely rotation bon

52、ds 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 第48页/共74页More Complex Splitting Patterns simplification of signal splitting occurs when coupling constants are the same第49页/共74页More Complex Splitting Patterns an example of p

53、eak 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 distinguishable第50页/共74页Stereochemistry & Topicity Depending on the symmetry of a molecule, otherw

54、ise equivalent hydrogens may be homotopic enantiotopic diastereotopic The 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 possible第51页/共74页Stereochemistry &

55、; Topicity Homotopic atoms or groups homotopic atoms or groups have identical chemical shifts under all conditionsAchiralHCHClClHCDClClDichloro-methane(achiral)Substitution does not produce a stereocenter;therefore hydrogensare homotopic.Substitute one H by DAchiralHCHClClHCDClClDichloro-methane(ach

56、iral)Substitution does not produce a stereocenter;therefore hydrogensare homotopic.Substitute one H by D第52页/共74页Stereochemistry & Topicity Enantiotopic groups enantiotopic atoms or groups have identical chemical shifts in achiral environments they have different chemical shifts in chiral enviro

57、nmentsChiralHCHClFHCDClFChlorofluoro-methane(achiral)Substitute one H by DSubstitution produces a stereocenter;therefore, hydrogens are enantiotopic. Both hydrogens are prochiral; one is pro-R-chiral, the other is pro-S-chiral.ChiralHCHClFHCDClFChlorofluoro-methane(achiral)Substitute one H by DSubst

58、itution produces a stereocenter;therefore, hydrogens are enantiotopic. Both hydrogens are prochiral; one is pro-R-chiral, the other is pro-S-chiral.第53页/共74页Stereochemistry & Topicity Diastereotopic groups H atoms on C-3 of 2-butanol are diastereotopic substitution by deuterium creates a chiral

59、center because there is already a chiral center in the molecule, diastereomers are now possible diastereotopic hydrogens have different chemical shifts under all conditionsHOHHHHOHHDChiralChiralSubstitute one Substitute one H on CHH on CH2 2 by D by D2-Butanol2-Butanol(chiral)(chiral)第54页/共74页Stereo

60、chemistry & Topicity The methyl groups on carbon 3 of 3-methyl-2-butanol are diastereotopic if a methyl hydrogen of carbon 4 is substituted by deuterium, a new chiral center is created because there is already one chiral center, diastereomers are now possible protons of the methyl groups on carbon 3 have differen