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 Websiteprotons. 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 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 vic￾inal 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 pro￾ton into a quartet. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website