《有机化学》课程教学资源（教材文献，英文版）CHAPTER 13 SPECTROSCOPY

CHAPTER THIRTEEN Spectroscopy ample rt oscillator NMR Spectrum 13.5 Diagram of a nuclear magnetic resonance spectrometer(From S. H. Pine, J.B. son, D J Cram, and G S Hammond, Organic Chemistry, 4th edition, McGraw-Hill, New 980p.136) It turns out though that there are several possible variations on this general theme. We could, for example, keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states. Or we could keep the rf constant and adjust the energy levels by varying the magnetic field strength. Both methods work, and the instruments based on them are called continuous wave(CW) spectrometers. Many of the terms we use in NMR spectroscopy have their origin in the way CW instruments operate, but Cw instruments are rarely used anymore CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transform nuclear magnetic resonance(FT-NMR) spectrometers FT-NMR spectrometers are far more versatile than CW instruments and are more com- plicated. Most of the visible differences between them lie in computerized data acquisi- tion and analysis components that are fundamental to FT-NMR spectroscopy. But there is an important difference in how a pulsed FT-NMR experiment is carried out as well Rather than sweeping through a range of frequencies(or magnetic field strengths ), the sample is irradiated with a short, intense burst of radiofrequency radiation(the pulse that excites all of the protons in the molecule. The magnetic field associated with the new orientation of nuclear spins induces an electrical signal in the receiver that decreases with time as the nuclei return to their original orientation. The resulting free-induction decay(FID) is a composite of the decay patterns of all of the protons in the molecule The free-induction decay pattern is stored in a computer and converted into a spectrum Richard r. ernst of the Swiss by a mathematical process known as a Fourier transform. The pulse-relaxation sequence Federal Institute of Technol- takes only about a second, but usually gives signals too weak to distinguish from back ground noise. The signal-to-noise ratio is enhanced by repeating the sequence many Prize in chemistry for devis- ing pulse-relaxation NMR times, then averaging the data. Noise is random and averaging causes it to vanish; sig nals always appear at the same place and accumulate. All of the operationsthe inter val between pulses, collecting, storing, and averaging the data and converting it to a pectrum by a Fourier transformare under computer control, which makes the actual taking of an FT-NMR spectrum a fairly routine operation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

It turns out though that there are several possible variations on this general theme. We could, for example, keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states. Or, we could keep the rf constant and adjust the energy levels by varying the magnetic field strength. Both methods work, and the instruments based on them are called continuous wave (CW) spectrometers. Many of the terms we use in NMR spectroscopy have their origin in the way CW instruments operate, but CW instruments are rarely used anymore. CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometers. FT-NMR spectrometers are far more versatile than CW instruments and are more com￾plicated. Most of the visible differences between them lie in computerized data acquisi￾tion and analysis components that are fundamental to FT-NMR spectroscopy. But there is an important difference in how a pulsed FT-NMR experiment is carried out as well. Rather than sweeping through a range of frequencies (or magnetic field strengths), the sample is irradiated with a short, intense burst of radiofrequency radiation (the pulse) that excites all of the protons in the molecule. The magnetic field associated with the new orientation of nuclear spins induces an electrical signal in the receiver that decreases with time as the nuclei return to their original orientation. The resulting free-induction decay (FID) is a composite of the decay patterns of all of the protons in the molecule. The free-induction decay pattern is stored in a computer and converted into a spectrum by a mathematical process known as a Fourier transform. The pulse-relaxation sequence takes only about a second, but usually gives signals too weak to distinguish from back￾ground noise. The signal-to-noise ratio is enhanced by repeating the sequence many times, then averaging the data. Noise is random and averaging causes it to vanish; sig￾nals always appear at the same place and accumulate. All of the operations—the inter￾val between pulses, collecting, storing, and averaging the data and converting it to a spectrum by a Fourier transform—are under computer control, which makes the actual taking of an FT-NMR spectrum a fairly routine operation. 492 CHAPTER THIRTEEN Spectroscopy Magnet rf input oscillator rf output signal amplifier NMR Spectrum rf output receiver rf input coil Sample tube 0 FIGURE 13.5 Diagram of a nuclear magnetic resonance spectrometer. (From S. H. Pine, J. B. Hendrickson, D. J. Cram, and G. S. Hammond, Organic Chemistry, 4th edition, McGraw-Hill, New York, 1980, p. 136.) Richard R. Ernst of the Swiss Federal Institute of Technol￾ogy won the 1991 Nobel Prize in chemistry for devis￾ing pulse-relaxation NMR techniques. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

13.4 Nuclear Shielding and H Chemical Shift Not only is pulsed FT-NMR the best method for obtaining proton spectra, it is the only practical method for many other nuclei, including C. It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy 13.4 NUCLEAR SHIELDING AND TH CHEMICAL SHIFTS Our discussion so far has concerned H nuclei in general without regard for the envi- other atoms--carbon, oxygen, nitrogen, and so on-by covalent bonds. The electrons in magnetic field of the elec the protons. Alone, a proton would feel the full strength of the external field, but a pro- gen bond opposes the - hydro these bonds, indeed all the electrons in a molecule, affect the magnetic environment of trons in the carbo ton in an organic molecule responds to both the external field plus any local fields within resulting magnetic field the molecule. An external magnetic field affects the motion of the electrons in a mole- experienced by the proton and the carbon is slightly cule, inducing local fields characterized by lines of force that circulate in the opposite less than se direction from the applied field(Figure 13. 6). Thus, the net field felt by a proton in a ess than %o- molecule will always be less than the applied field, and the proton is said to be shielded All of the protons of a molecule are shielded from the applied field by the electrons, but some are less shielded than others Sometimes the term"deshielded is used to describ this decreased shielding of one proton relative to another. The more shielded a proton is, the greater must be the strength of the applied field in order to achieve resonance and produce a signal. A more shielded proton absorbs rf radiation at higher field strength (upfield) compared with one at lower field strength (downfield). Different protons give signals at different field strengths. The dependence of the resonance position of a nucleus that results from its molecular environment is called its chemical shift. This is where the real power of NMR lies. The chemical shifts of various protons in a molecule can be different and are characteristic of particular struc tural features the igure 13.7 shows the H NMR spectrum of chloroform(CHCl3)to illustrate how ology just developed applies to a real Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard--tetramethylsilane( CH3)4 Si, abbreviated TMS. The protons of Tms are more shielded than those of most organic compounds, so all of the signals in a sam- ple ordinarily appear at lower field than those of the TMS reference. When measured this chapter is an ele using a 100-MHz instrument, the signal for the proton in chloroform(CHCI,), for exam- pane tearing ay modeling con. pIe, appears 728 Hz downfield from the TMS signal. But since frequency is proportional tains models of (CHa) Si and to magnetic field strength, the same signal would appear 1456 Hz downfield from TMS (CHa)aC in which the greate on a 200-MHz instrument. We simplify the reporting of chemical shifts by converting and hydrogens of TMS is appar need not actually be present in the sample, nor even appear in the spectrum in order to tential and in the calculated o them to parts per million(ppm) from TMS, which is assigned a value of 0. The TMs ent be serve as a reference Chemical shift(8)= position of signal- position of TMS peak x 106 spectrometer frequency Thus, the chemical shift for the proton in chloroform is: 1456Hz-0Hz 200×10°Hz ×10=728pm When chemical shifts are reported this way, they are identified by the symbol 8 and are independent of the field strength. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Not only is pulsed FT-NMR the best method for obtaining proton spectra, it is the only practical method for many other nuclei, including 13C. It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy. 13.4 NUCLEAR SHIELDING AND 1 H CHEMICAL SHIFTS Our discussion so far has concerned 1 H nuclei in general without regard for the envi￾ronments of individual protons in a molecule. Protons in a molecule are connected to other atoms—carbon, oxygen, nitrogen, and so on—by covalent bonds. The electrons in these bonds, indeed all the electrons in a molecule, affect the magnetic environment of the protons. Alone, a proton would feel the full strength of the external field, but a pro￾ton in an organic molecule responds to both the external field plus any local fields within the molecule. An external magnetic field affects the motion of the electrons in a mole￾cule, inducing local fields characterized by lines of force that circulate in the opposite direction from the applied field (Figure 13.6). Thus, the net field felt by a proton in a molecule will always be less than the applied field, and the proton is said to be shielded. All of the protons of a molecule are shielded from the applied field by the electrons, but some are less shielded than others. Sometimes the term “deshielded,” is used to describe this decreased shielding of one proton relative to another. The more shielded a proton is, the greater must be the strength of the applied field in order to achieve resonance and produce a signal. A more shielded proton absorbs rf radiation at higher field strength (upfield) compared with one at lower field strength (downfield). Different protons give signals at different field strengths. The dependence of the resonance position of a nucleus that results from its molecular environment is called its chemical shift. This is where the real power of NMR lies. The chemical shifts of various protons in a molecule can be different and are characteristic of particular struc￾tural features. Figure 13.7 shows the 1 H NMR spectrum of chloroform (CHCl3) to illustrate how the terminology just developed applies to a real spectrum. Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard—tetramethylsilane (CH3)4Si, abbreviated TMS. The protons of TMS are more shielded than those of most organic compounds, so all of the signals in a sam￾ple ordinarily appear at lower field than those of the TMS reference. When measured using a 100-MHz instrument, the signal for the proton in chloroform (CHCl3), for exam￾ple, appears 728 Hz downfield from the TMS signal. But since frequency is proportional to magnetic field strength, the same signal would appear 1456 Hz downfield from TMS on a 200-MHz instrument. We simplify the reporting of chemical shifts by converting them to parts per million (ppm) from TMS, which is assigned a value of 0. The TMS need not actually be present in the sample, nor even appear in the spectrum in order to serve as a reference. Chemical shift ()  106 Thus, the chemical shift for the proton in chloroform is:   106 7.28 ppm When chemical shifts are reported this way, they are identified by the symbol  and are independent of the field strength. 1456 Hz 0 Hz 200  106 Hz position of signal position of TMS peak spectrometer frequency 13.4 Nuclear Shielding and 1 H Chemical Shifts 493 0 C H FIGURE 13.6 The induced magnetic field of the elec￾trons in the carbon–hydro￾gen bond opposes the exter￾nal magnetic field. The resulting magnetic field experienced by the proton and the carbon is slightly less than 0. The graphic that begins this chapter is an electrostatic potential map of tetramethylsi￾lane. Learning By Modeling con￾tains models of (CH3)4Si and (CH3)4C in which the greater electron density at the carbons and hydrogens of TMS is appar￾ent both in the electrostatic po￾tential and in the calculated atomic charges. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER THIRTEEN Spectroscopy 87.28 ppm Downfield Decreased shielding Upfield Increased shielding CTMS) 80 ppm Chemical shift(8, ppm) FIGURE 13.7 The 200-MHz 'H NMR spectrum of chloroform(HCCl3). Chemical shifts are mea red along the x-axis in parts per million (ppm) from tetramethylsilane as the reference, wh is assigned a value of zero. PROBLEM 13.3 The 'H NMR signal for bromoform(CHBr3) appears at 2065 Hz when recorded on a 300-MHz NMR spectrometer. (a) What is the chemical shift of this proton?(b)Is the proton in CHBr3 more shielded or less shielded than the NMR spectra are usually run in solution and, although chloroform is a good sol vent for most organic compounds, it's rarely used because its own signal at 8 7.28 ppm would be so intense that it would obscure signals in the sample. Because the magnetic properties of deuterium(D=H)are different from those of H, CDCl3 gives no sig nals at all in an H NMR spectrum and is used instead. Indeed, CDCl3 is the most com- monly used solvent in H NMR spectroscopy. Likewise, D2O is used instead of H2O for water-soluble substances such as carbohydrates 13.5 EFFECTS OF MOLECULAR STRUCTURE ON TH CHEMICAL SHIFTS Nuclear magnetic resonance spectroscopy is such a powerful tool for structure determi Problem 13.3 in the preced- nation because protons in different environments experience different degrees of shield- on the ing and have different chemical shifts. In compounds of the type CH3X, for example, chemical shift difference be. the shielding of the methyl protons increases as X becomes less electronegative. Inas Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

PROBLEM 13.3 The 1 H NMR signal for bromoform (CHBr3) appears at 2065 Hz when recorded on a 300-MHz NMR spectrometer. (a) What is the chemical shift of this proton? (b) Is the proton in CHBr3 more shielded or less shielded than the proton in CHCl3? NMR spectra are usually run in solution and, although chloroform is a good sol￾vent for most organic compounds, it’s rarely used because its own signal at  7.28 ppm would be so intense that it would obscure signals in the sample. Because the magnetic properties of deuterium (D 2 H) are different from those of 1 H, CDCl3 gives no sig￾nals at all in an 1 H NMR spectrum and is used instead. Indeed, CDCl3 is the most com￾monly used solvent in 1 H NMR spectroscopy. Likewise, D2O is used instead of H2O for water-soluble substances such as carbohydrates. 13.5 EFFECTS OF MOLECULAR STRUCTURE ON 1 H CHEMICAL SHIFTS Nuclear magnetic resonance spectroscopy is such a powerful tool for structure determi￾nation because protons in different environments experience different degrees of shield￾ing and have different chemical shifts. In compounds of the type CH3X, for example, the shielding of the methyl protons increases as X becomes less electronegative. Inas- 494 CHAPTER THIRTEEN Spectroscopy Problem 13.3 in the preced￾ing section was based on the chemical shift difference be￾tween the proton in CHCl3 and the proton in CHBr3 and its relation to shielding. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Chemical shift (δ, ppm) Tetramethylsilane (TMS) δ 0 ppm H±CCl3 δ 7.28 ppm Upfield Increased shielding Downfield Decreased shielding FIGURE 13.7 The 200-MHz 1 H NMR spectrum of chloroform (HCCl3). Chemical shifts are mea￾sured along the x-axis in parts per million (ppm) from tetramethylsilane as the reference, which is assigned a value of zero. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

13.5 Effects of molecular Structure on 'H Chemical Shifts much as the shielding is the electrons, it isnt surprising to find that the chemi- cal shift depends on the de to which X draws electrons away from the methyl group Increased shielding of methyl protons Decreasing electronegativity of attached atom CH3OCH3 (CH3)3N CH3CH Methyl Dimethyl Trimethylamine Ethane fluoride Chemical shift of methyl protons (6),ppm: A similar trend is seen in the methyl halides, in which the protons in CH3 F are the least shielded(8 4.3 ppm) and those of CH3I (8 2. 2 ppm) are the most The deshielding effects of electronegative substituents are cumulative, as the chem- ical shifts for various chlorinated derivatives of methane indicate CH,CI CH3CI Chloroform Methylene chloride Methyl chloride Chemical shift (6),ppm: 7.3 PROBLEM 13. 4 There is a difference of 4.6 ppm in the 'H chemical shifts CHCI3 and CH3CCl3 What is the chemical shift for the protons in CH3 CCl3? Exp your reasonIng inyl protons in alkenes and aryl protons in arenes are substantially less shielded Ethylene Chemical shift 7.3 One reason for the decreased shielding of vinyl and aryl protons is related to the directional properties of the induced magnetic field of the TT electrons. As Figure 13.8 shows, the induced magnetic field due to the T electrons is just like that due to elec trons in o bonds; it opposes the applied magnetic field. However, all magnetic fields close upon themselves, and protons attached to a carbon-carbon double bond or ar FIGURE 13. 8 The induced matic ring lie in a region where the induced field reinforces the applied field, which magnetic field of the elec- decreases the shielding of vinyl and aryl protons trons of (a) an alkene and A similar, although much smaller, effect of T electron systems is seen in the chem- (b)an arene reinforces the ical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylben- where vinyl and aryl protons zene and in 2, 3-dimethyl-2-butene are less shielded than those in ethane are located Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

much as the shielding is due to the electrons, it isn’t surprising to find that the chemi￾cal shift depends on the degree to which X draws electrons away from the methyl group. A similar trend is seen in the methyl halides, in which the protons in CH3F are the least shielded ( 4.3 ppm) and those of CH3I ( 2.2 ppm) are the most. The deshielding effects of electronegative substituents are cumulative, as the chem￾ical shifts for various chlorinated derivatives of methane indicate: PROBLEM 13.4 There is a difference of 4.6 ppm in the 1 H chemical shifts of CHCl3 and CH3CCl3. What is the chemical shift for the protons in CH3CCl3? Explain your reasoning. Vinyl protons in alkenes and aryl protons in arenes are substantially less shielded than protons in alkanes: One reason for the decreased shielding of vinyl and aryl protons is related to the directional properties of the induced magnetic field of the electrons. As Figure 13.8 shows, the induced magnetic field due to the electrons is just like that due to elec￾trons in  bonds; it opposes the applied magnetic field. However, all magnetic fields close upon themselves, and protons attached to a carbon–carbon double bond or an aro￾matic ring lie in a region where the induced field reinforces the applied field, which decreases the shielding of vinyl and aryl protons. A similar, although much smaller, effect of electron systems is seen in the chem￾ical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylben￾zene and in 2,3-dimethyl-2-butene are less shielded than those in ethane. Chemical shift (), ppm: H H H H H H Benzene 7.3 C H H H H C Ethylene 5.3 CH3CH3 Ethane 0.9 Chemical shift (), ppm: CHCl3 Chloroform (trichloromethane) 7.3 CH2Cl2 Methylene chloride (dichloromethane) 5.3 CH3Cl Methyl chloride (chloromethane) 3.1 Increased shielding of methyl protons Decreasing electronegativity of attached atom Chemical shift of methyl protons (), ppm: CH3F Methyl fluoride 4.3 CH3OCH3 Dimethyl ether 3.2 (CH3)3N Trimethylamine 2.2 CH3CH3 Ethane 0.9 13.5 Effects of Molecular Structure on 1 H Chemical Shifts 495 C C H H H H H H H H H H 0 (a) (b) 0 FIGURE 13.8 The induced magnetic field of the elec￾trons of (a) an alkene and (b) an arene reinforces the applied fields in the regions where vinyl and aryl protons are located. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER THIRTEEN Spectroscopy H3C H3C H3C H2 Hexamethylbenzene 2, 3-Dimethyl-2-butene Chemical shift (6),ppm: Table 13.1 collects chemical-shift information for protons of various types. Within each type, methyl(CH3)protons are more shielded than methylene(CH2) protons, and methylene protons are more shielded than methine(Ch) protons. These differences are mall--only about 0.7 ppm separates a methyl proton from a methine proton of the same type. Overall, proton chemical shifts among common organic compounds encompass a range of about 12 ppm. The protons in alkanes are the most shielded, and O-H pro- tons of carboxylic acids are the least shielded TABLE 13.1 Chemical Shifts of Representative Types of Protons Chemical shift(6) Chemical shift(6) Type of proton ppm* Type of proton ppm* 0.9-1.8 2.2-2.9 1.6-2.6 3.1-4.1 2.1-2.5 2.7-4.1 H-C-C≡N 3.3-3.7 2.5 2.3-2.8 H-NR 4.5-6.5 6.5-8.5 H— H-oc- 10-1 *Approximate values relative to tetramethylsilane; other groups within the molecule can cause a proton The chemical shifts of protons bonded to nitrogen and oxygen are temperature- and concentration- Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Table 13.1 collects chemical-shift information for protons of various types. Within each type, methyl (CH3) protons are more shielded than methylene (CH2) protons, and methylene protons are more shielded than methine (CH) protons. These differences are small—only about 0.7 ppm separates a methyl proton from a methine proton of the same type. Overall, proton chemical shifts among common organic compounds encompass a range of about 12 ppm. The protons in alkanes are the most shielded, and O±H pro￾tons of carboxylic acids are the least shielded. Chemical shift (), ppm: CH3 CH3 H3C CH3 H3C H3C Hexamethylbenzene 2.2 C CH3 CH3 H3C H3C C 2,3-Dimethyl-2-butene 1.7 496 CHAPTER THIRTEEN Spectroscopy TABLE 13.1 Chemical Shifts of Representative Types of Protons *Approximate values relative to tetramethylsilane; other groups within the molecule can cause a proton signal to appear outside of the range cited. † The chemical shifts of protons bonded to nitrogen and oxygen are temperature- and concentration￾dependent. Type of proton H±C±R W W H±C±CœC W W H±C±CPN W W H±CPC± H±C±C± W W O X H±C± O X H±C±Ar W W H±Ar H±CœC ± ± W Type of proton H±C±NR W W H±C±Cl W W H±C±Br W W H±C±O W W H±OC± O X H±NR H±OAr H±OR 0.9–1.8 Chemical shift (), ppm* 1.6–2.6 2.1–2.5 2.1–3 2.5 2.3–2.8 4.5–6.5 6.5–8.5 9–10 2.2–2.9 Chemical shift (), ppm* 3.1–4.1 2.7–4.1 3.3–3.7 1–3† 0.5–5† 6–8† 10–13† Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website