3.7 Spin-Spin Splitting in NMR Spectroscopy H3 Cl2CHCH3 8.0 5.0 Chemical shift(8, ppm) FIGURE 13. 10 The 200-MHz H NMR spectrum of 1, 1-dichloroethane, showing the methine roton as a quartet and the methyl protons as a doublet. the peak multiplicities are seen more moment of the methine proton is parallel to the applied field, it reinforces it. This decreases the shielding of the methyl protons and causes their signal to appear at slightly lower field strength. Conversely, when the magnetic moment of the methine proton is antiparallel to the applied field, it opposes it and increases the shielding of the methyl protons. Instead of a single peak for the methyl protons, there are two of approximately qual intensity: one at slightly higher field than the"true"chemical shift, the other at slightly lower field Turning now to the methine proton, its signal is split by the methyl protons into a quartet. The same kind of analysis applies here and is outlined in Figure 13.12. The methine proton"sees"eight different combinations of nuclear spins for the methyl FIGURE 13. 11 The magnetic moments (blue arrows) of the two possible spin states of the methine proton affect the chemical shift of the H-C-Cl dichloroethane. When the magnetic moment is parallel the external field external field and a smalle geo is needed for resonance When it is antiparallel to the of methin reinforces t hine proton shields external field, it subtracts a weaker lo is needed for resonance. methyl protons from Co- Methyl signal appears a Methyl signal appears at higher field. methyl protons. Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitemoment of the methine proton is parallel to the applied field, it reinforces it. This decreases the shielding of the methyl protons and causes their signal to appear at slightly lower field strength. Conversely, when the magnetic moment of the methine proton is antiparallel to the applied field, it opposes it and increases the shielding of the methyl protons. Instead of a single peak for the methyl protons, there are two of approximately equal intensity: one at slightly higher field than the “true” chemical shift, the other at slightly lower field. Turning now to the methine proton, its signal is split by the methyl protons into a quartet. The same kind of analysis applies here and is outlined in Figure 13.12. The methine proton “sees” eight different combinations of nuclear spins for the methyl 13.7 Spin–Spin Splitting in NMR Spectroscopy 501 4.0 3.0 2.0 1.0 0.0 Chemical shift (δ, ppm) 8.0 7.0 6.0 6.4 6.0 5.6 2.4 2.0 9.0 5.0 CH3 Cl2CH Cl2CHCH3 FIGURE 13.10 The 200-MHz 1 H NMR spectrum of 1,1-dichloroethane, showing the methine proton as a quartet and the methyl protons as a doublet. The peak multiplicities are seen more clearly in the scale-expanded insets. Cl W W CH3 0 H±C±Cl Spin of methine proton reinforces 0; a weaker 0 is needed for resonance. Methyl signal appears at lower field. Spin of methine proton shields methyl protons from 0. Methyl signal appears at higher field. Cl W W CH3 H±C±Cl FIGURE 13.11 The magnetic moments (blue arrows) of the two possible spin states of the methine proton affect the chemical shift of the methyl protons in 1,1- dichloroethane. When the magnetic moment is parallel to the external field 0 (green arrow), it adds to the external field and a smaller 0 is needed for resonance. When it is antiparallel to the external field, it subtracts from it and shields the methyl protons. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website