CHAPTER THIRTEEN Spectroscopy ↑44↑↑*+++++ - -3J There are eight These eight combinations cause the of the nuclear spins of the three methyl gnal of the CHCl2 proton to b protons in CHiCHCI tensities of the peaks are in the FIGURE 13.12 nethyl protons of 1, 1-dichloroethane split the signal of the methine pro- protons. In one combination, the magnetic moments of all three methyl protons rein- force the applied field. At the other extreme, the magnetic moments of all three methyl protons oppose the applied field. There are three combinations in which the magnetic moments of two methyl protons reinforce the applied field, whereas one opposes it. Finally, there are three combinations in which the magnetic moments of two methyl protons oppose the applied field and one reinforces it. These eight possible combina- tions give rise to four distinct peaks for the methine proton, with a ratio of intensities of1:3:3:1 We describe the observed splitting of NMR signals as spin-spin splitting and the physical basis for it as spin-spin coupling. It has its origin in the communication of nuclear spin information between nuclei. This information is transmitted by way of the electrons in the bonds that intervene between the nuclei. Its effect is greatest when the number of bonds is small. Vicinal protons are separated by three bonds, and coupling between vicinal protons, as in 1, I-dichloroethane, is called three-bond coupling or vic nal coupling. Four-bond couplings are weaker and not normally observable A very important characteristic of spin-spin splitting is that protons that have the same chemical shift do not split each other's signal. Ethane, for example, shows only a single sharp peak in its NMR spectrum. Even though there is a vicinal relationship between the protons of one methyl group and those of the other, they do not split each other's signal because they are equivalent PROBLEM 13.8 Describe the appearance of the 'H NMR spectrum of each of the following compounds. How many signals would you expect to find, and into how many peaks will each signal be split? (a)1, 2-Dichloroethane (d)1, 2, 2-Trichloropropane (b)1, 1, 1-Trichloroethane ( e)1,1, 1, 2-Tetrachloropropane SAMPLE SoLUTION (a)All the protons of 1, 2-dichloroethane(CICH2 CH2 CI)are chemically equivalent and have the same chemical shift. Protons that have the same chemical shift do not split each other's signal, and so the NMR spectrum of 1, 2-dichloroethane consists of a single sharp peak Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
protons. In one combination, the magnetic moments of all three methyl protons reinforce the applied field. At the other extreme, the magnetic moments of all three methyl protons oppose the applied field. There are three combinations in which the magnetic moments of two methyl protons reinforce the applied field, whereas one opposes it. Finally, there are three combinations in which the magnetic moments of two methyl protons oppose the applied field and one reinforces it. These eight possible combinations give rise to four distinct peaks for the methine proton, with a ratio of intensities of 1:3:3:1. We describe the observed splitting of NMR signals as spin–spin splitting and the physical basis for it as spin–spin coupling. It has its origin in the communication of nuclear spin information between nuclei. This information is transmitted by way of the electrons in the bonds that intervene between the nuclei. Its effect is greatest when the number of bonds is small. Vicinal protons are separated by three bonds, and coupling between vicinal protons, as in 1,1-dichloroethane, is called three-bond coupling or vicinal coupling. Four-bond couplings are weaker and not normally observable. A very important characteristic of spin–spin splitting is that protons that have the same chemical shift do not split each other’s signal. Ethane, for example, shows only a single sharp peak in its NMR spectrum. Even though there is a vicinal relationship between the protons of one methyl group and those of the other, they do not split each other’s signal because they are equivalent. PROBLEM 13.8 Describe the appearance of the 1 H NMR spectrum of each of the following compounds. How many signals would you expect to find, and into how many peaks will each signal be split? (a) 1,2-Dichloroethane (d) 1,2,2-Trichloropropane (b) 1,1,1-Trichloroethane (e) 1,1,1,2-Tetrachloropropane (c) 1,1,2-Trichloroethane SAMPLE SOLUTION (a) All the protons of 1,2-dichloroethane (ClCH2CH2Cl) are chemically equivalent and have the same chemical shift. Protons that have the same chemical shift do not split each other’s signal, and so the NMR spectrum of 1,2-dichloroethane consists of a single sharp peak. 502 CHAPTER THIRTEEN Spectroscopy There are eight possible combinations of the nuclear spins of the three methyl protons in CH3CHCl2. These eight combinations cause the signal of the CHCl2 proton to be split into a quartet, in which the intensities of the peaks are in the ratio 1:3:3:1. 3 Jab 3 Jab 3 Jab FIGURE 13.12 The methyl protons of 1,1-dichloroethane split the signal of the methine proton into a quartet. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
13.8 Splitting Patterns: The Ethyl Group Coupling of nuclear spins requires that the nuclei split each other's signal equally The separation between the two halves of the methyl doublet in 1, 1-dichloroethane is qual to the separation between any two adjacent peaks of the methine quartet. The extent to which two nuclei are coupled is known as the coupling constant J and in simple cases is equal to the separation between adjacent lines of the signal of a particular pre ton. The three-bond coupling constant Jab in 1, l-dichloroethane has a value of 7 Hz The size of the coupling constant is independent of the field strength, the separation between adjacent peaks in 1, 1-dichloroethane is 7 Hz, irrespective of whether the spec trum is recorded at 200 MHz or 500 Mhz 13. 8 SPLITTING PATTERNS: THE ETHYL GROUP At first glance, splitting may seem to complicate the interpretation of NMR spectra. In fact, it makes structure determination easier because it provides additional information It tells us how many protons are vicinal to a proton responsible for a particular signal. With practice, we learn to pick out characteristic patterns of peaks, associating them with particular structural types. One of the most common of these patterns is that of the ethyl group, represented in the NMR spectrum of ethyl bromide in Figure 13 13 H3 801.701.60 CH, BrCh,ci CHCI3 TMS 3.0 0.0 Chemical shift(8, Ppm) FIGURE 13 13 The 200-MHz H NMR spectrum of ethyl bromide showing the characteristic triplet-quartet pattern of an ethyl group Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Coupling of nuclear spins requires that the nuclei split each other’s signal equally. The separation between the two halves of the methyl doublet in 1,1-dichloroethane is equal to the separation between any two adjacent peaks of the methine quartet. The extent to which two nuclei are coupled is known as the coupling constant J and in simple cases is equal to the separation between adjacent lines of the signal of a particular proton. The three-bond coupling constant 3 Jab in 1,1-dichloroethane has a value of 7 Hz. The size of the coupling constant is independent of the field strength; the separation between adjacent peaks in 1,1-dichloroethane is 7 Hz, irrespective of whether the spectrum is recorded at 200 MHz or 500 MHz. 13.8 SPLITTING PATTERNS: THE ETHYL GROUP At first glance, splitting may seem to complicate the interpretation of NMR spectra. In fact, it makes structure determination easier because it provides additional information. It tells us how many protons are vicinal to a proton responsible for a particular signal. With practice, we learn to pick out characteristic patterns of peaks, associating them with particular structural types. One of the most common of these patterns is that of the ethyl group, represented in the NMR spectrum of ethyl bromide in Figure 13.13. 13.8 Splitting Patterns: The Ethyl Group 503 4.0 3.0 2.0 1.0 0.0 Chemical shift (δ, ppm) 9.0 8.0 7.0 6.0 3.6 3.5 3.4 3.3 1.80 1.70 1.60 5.0 CHCl3 TMS BrCH2CH3 CH3 CH2 FIGURE 13.13 The 200-MHz 1 H NMR spectrum of ethyl bromide, showing the characteristic triplet–quartet pattern of an ethyl group. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER THIRTEEN Spectroscopy In compounds of the type CH3CH2X, especially where X is an electronegative atom +++++ or group, such as bromine in ethyl bromide, the ethyl group appears as a tripler-quartet pattern. The methylene proton signal is split into a quartet by coupling with the methyl binations of the nuclear protons. The signal for the methyl protons is a triplet because of vicinal coupling to the of the two methylene two protons of the adjacent methylene group protons in CH3 CH2B1 H2—CH3 These two protons split These three protons split the methyl signal into the methylene signal into a quartet. le have discussed in the preceding section why methyl groups split the si due to vicinal protons into a quartet. Splitting by a methylene group gives a triplet cor- responding to the spin combinations shown in Figure 13 14 for ethyl bromide. The rel- ative intensities of the peaks of this triplet are 1: 2: 1 PROBLEM 13.9 Describe the appearance of the H NMR spectrum of each of the These four combinations cause he signal of the CHs protons to following compounds. How many signals would you expect to find, and into how many peaks will each signal be split? the intensities of the peaks are (a)CICH2 OCH2 CH3 in the ratio 1: 2: 1 (b)CH3 CH2OCH3 FIGURE 13 14 The methyl- (c)CHaCH2OCH2 CH3 ene protons of ethyl bro- ( d) p-Diethylbenzene mide split the signal of the (e)CICH2 CH, CH SAMPLE SOLUTION (a)Along with the triplet-quartet pattern of the ethyl group, the NMr spectrum of this compound will contain a singlet for the two protons of the chloromethyl group Split into triplet by two ClCH2-0--CH2--CH3 "z- protons of adjacent Singlet: no protons- methylene group vicinal to the Split into quartet by herefore, no splitting three protons of methyl group Table 13.2 summarizes the splitting patterns and peak inter pling to various numbers of protons TABLE 13.2 Splitting Patterns of Common Multi triplets Number of equivalent protons Appearance of Intensities of lines to which nucleus is coupled in multiplet expansion(Pascals triangle Quartet 3:3:1 Pentet 1:5:10:10:5:1 Septet 1:6:15:20:15:6:1 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
In compounds of the type CH3CH2X, especially where X is an electronegative atom or group, such as bromine in ethyl bromide, the ethyl group appears as a triplet–quartet pattern. The methylene proton signal is split into a quartet by coupling with the methyl protons. The signal for the methyl protons is a triplet because of vicinal coupling to the two protons of the adjacent methylene group. We have discussed in the preceding section why methyl groups split the signals due to vicinal protons into a quartet. Splitting by a methylene group gives a triplet corresponding to the spin combinations shown in Figure 13.14 for ethyl bromide. The relative intensities of the peaks of this triplet are 1:2:1. PROBLEM 13.9 Describe the appearance of the 1 H NMR spectrum of each of the following compounds. How many signals would you expect to find, and into how many peaks will each signal be split? (a) ClCH2OCH2CH3 (b) CH3CH2OCH3 (c) CH3CH2OCH2CH3 (d) p-Diethylbenzene (e) ClCH2CH2OCH2CH3 SAMPLE SOLUTION (a) Along with the triplet–quartet pattern of the ethyl group, the NMR spectrum of this compound will contain a singlet for the two protons of the chloromethyl group. Table 13.2 summarizes the splitting patterns and peak intensities expected for coupling to various numbers of protons. Split into triplet by two protons of adjacent methylene group Split into quartet by three protons of methyl group Singlet; no protons vicinal to these; therefore, no splitting ClCH2 O CH2 CH3 Br CH2 CH3 These three protons split the methylene signal into a quartet. These two protons split the methyl signal into a triplet. 504 CHAPTER THIRTEEN Spectroscopy There are four possible combinations of the nuclear spins of the two methylene protons in CH3CH2Br. 3Jab 3Jab These four combinations cause the signal of the CH3 protons to be split into a triplet, in which the intensities of the peaks are in the ratio 1:2:1. FIGURE 13.14 The methylene protons of ethyl bromide split the signal of the methyl protons into a triplet. TABLE 13.2 Splitting Patterns of Common Multiplets Number of equivalent protons to which nucleus is coupled 1 2 3 4 5 6 1:1 1:2:1 1:3:3:1 1:4 :6:4 :1 1:5:10:10:5:1 1:6:15:20:15:6:1 Intensities of lines in multiplet Doublet Triplet Quartet Pentet Sextet Septet Appearance of multiplet The intensities correspond to the coefficients of a binomial expansion (Pascal’s triangle). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
13.10 Splitting Patterns: Pairs of Doublets H FIGURE 13. 15 The 200-MHz H3C-C—CH3 propyl chloride, showing the an isopropyl group 100 0.0 Chemical shift(8, ppm) 13.9 SPLITTING PATTERNS: THE ISOPROPYL GROUP The NMR spectrum of isopropyl chlo gure 13. 15)illustrates the appearance of an isopropyl group. The signal for the six equivalent methyl protons at 8 1.5 ppm is split into a doublet by the proton of the H-C-Cl unit In turn, the H-C-Cl proton sig- nal at 84.2 ppm is split into a septet by the six methyl protons. a doublet-septet pat- tern is characteristic of an isopropyl group. This proton splits the / H、 CH These six signal for the methyl plit the me protons into a doublet. CI CH3 signal into 13.10 SPLITTING PATTERNS: PAIRS OF DOUBLETS We often see splitting patterns in which the intensities of the individual peaks do not match those given in Table 13. 2, but are distorted in that the signals for coupled protons lean"toward each other. This leaning is a general phenomenon, but is most easily illus trated for the case of two nonequivalent vicinal protons as shown in Figure 13 16 HI- The appearance of the splitting pattern of protons I and 2 depends on their coupling con- stant J and the chemical shift difference Av between them. When the ratio Av/ is large, two symmetrical 1: I doublets are observed. We refer to this as the"AX case, using two Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
13.10 Splitting Patterns: Pairs of Doublets 505 4.0 3.0 2.0 1.0 0.0 Chemical shift (δ, ppm) 10.0 5.0 9.0 8.0 7.0 6.0 CH CH3 H W W Cl 4.4 4.3 4.2 4.1 4.0 H3C±C±CH3 1.8 1.6 1.4 FIGURE 13.15 The 200-MHz 1 H NMR spectrum of isopropyl chloride, showing the doublet–septet pattern of an isopropyl group. 13.9 SPLITTING PATTERNS: THE ISOPROPYL GROUP The NMR spectrum of isopropyl chloride (Figure 13.15) illustrates the appearance of an isopropyl group. The signal for the six equivalent methyl protons at 1.5 ppm is split into a doublet by the proton of the H±C±Cl unit. In turn, the H±C±Cl proton signal at 4.2 ppm is split into a septet by the six methyl protons. A doublet–septet pattern is characteristic of an isopropyl group. 13.10 SPLITTING PATTERNS: PAIRS OF DOUBLETS We often see splitting patterns in which the intensities of the individual peaks do not match those given in Table 13.2, but are distorted in that the signals for coupled protons “lean” toward each other. This leaning is a general phenomenon, but is most easily illustrated for the case of two nonequivalent vicinal protons as shown in Figure 13.16. H1±C±C±H2 The appearance of the splitting pattern of protons 1 and 2 depends on their coupling constant J and the chemical shift difference between them. When the ratio /J is large, two symmetrical 1:1 doublets are observed. We refer to this as the “AX” case, using two This proton splits the signal for the methyl protons into a doublet. These six protons split the methine signal into a septet. H CH3 CH3 C Cl Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER THIRTEEN Spectroscopy FIGURE 13 16 The appear- ance of the splitting pattern Chemical shift difference ons depends on their coupling much larger than coupling constant and the chemica hift difference Ay between them. As the ratio△wde reases. the doublets be come increasingly distorted When the two protons have the same chemical shift, no splitting is observed. Same chemical shift: pitting letters that are remote in the alphabet to stand for signals well removed from each other on the spectrum. Keeping the coupling constant the same while reducing Av leads to a steady decrease in the intensity of the outer two peaks with a simultaneous increase in the inner two as we progress from AX through AM to AB. At the extreme(A2), the two protons have the same chemical shift, the outermost lines have disappeared, and no split ting is observed. Because of its appearance, it is easy to misinterpret an AB pattern as a quartet, rather than the pair of skewed doublets it really is. The skewed AB pattern is clearly visible in the H NMR spectrum of 2,3,4- trichloroanisole(Figure 13. 17). In addition to the singlet at 8 3.9 ppm for the protons of the -OCH, group, we see doublets at 8 6.8 and 87.3 ppm for the two protons of the aromatic ring Doublet dOublet 87.3 ppm HH 86.8ppm -OCH,"sInglet 2.3.4-Trichloroanisole Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
letters that are remote in the alphabet to stand for signals well removed from each other on the spectrum. Keeping the coupling constant the same while reducing leads to a steady decrease in the intensity of the outer two peaks with a simultaneous increase in the inner two as we progress from AX through AM to AB. At the extreme (A2), the two protons have the same chemical shift, the outermost lines have disappeared, and no splitting is observed. Because of its appearance, it is easy to misinterpret an AB pattern as a quartet, rather than the pair of skewed doublets it really is. The skewed AB pattern is clearly visible in the 1 H NMR spectrum of 2,3,4- trichloroanisole (Figure 13.17). In addition to the singlet at 3.9 ppm for the protons of the ±OCH3 group, we see doublets at 6.8 and 7.3 ppm for the two protons of the aromatic ring. Doublet 7.3 ppm Doublet 6.8 ppm Singlet 3.9 ppm Cl OCH3 Cl Cl H H 2,3,4-Trichloroanisole 506 CHAPTER THIRTEEN Spectroscopy Chemical shift difference much larger than coupling constant AX A2 AM Same chemical shift; no splitting AB J J J J J J FIGURE 13.16 The appearance of the splitting pattern of two coupled protons depends on their coupling constant J and the chemical shift difference between them. As the ratio /J decreases, the doublets become increasingly distorted. When the two protons have the same chemical shift, no splitting is observed. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website