13.7 Spin-Spin Splitting in NMR Spectroscopy

1. 13.7Spin-Spin SplittinginNMR Spectroscopy not all peaks are singlets signals can be split by coupling of nuclear spins

2. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Cl2CHCH3 Figure 13.10 (page 501) 4 lines; quartet 2 lines; doublet CH3 CH Chemical shift (d, ppm)

3. C C C C Two-bond and three-bond coupling H H H H protons separated bytwo bonds(geminal relationship) protons separated bythree bonds(vicinal relationship)

4. C C C C Two-bond and three-bond coupling H in order to observe splitting, protons cannot have same chemical shift coupling constant (2J or 3J) is independent of field strength H H H

5. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Cl2CHCH3 Figure 13.10 (page 501) 4 lines; quartet 2 lines; doublet coupled protons are vicinal (three-bond coupling) CH splits CH3 into a doublet CH3 splits CH into a quartet CH3 CH Chemical shift (d, ppm)

6. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? To explain the splitting of the protons at C-2, we first focus on the two possible spin orientations of the proton at C-1 signal for methyl protons is split into a doublet

7. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? There are two orientations of the nuclear spin for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C-2 protons. signal for methyl protons is split into a doublet

8. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? The protons at C-2 "feel" the effect of both the applied magnetic field and the local field resulting from the spin of the C-1 proton. signal for methyl protons is split into a doublet

9. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? "true" chemicalshift of methylprotons (no coupling) this line correspondsto molecules in which the nuclear spin of the proton at C-1 reinforcesthe applied field this line correspondsto molecules in which the nuclear spin of the proton at C-1 opposesthe applied field

10. H Cl H H C C H Cl Why does the methine proton of1,1-dichloroethane appear as a quartet? The proton at C-1 "feels" the effect of the applied magnetic field and the local fields resulting from the spin states of the three methyl protons. The possible combinations are shown on the next slide. signal for methine proton is split into a quartet

11. H Cl H H C C H Cl Why does the methine proton of1,1-dichloroethane appear as a quartet? There are eight combinations of nuclear spins for the three methyl protons. These 8 combinations split the signal into a 1:3:3:1 quartet.

12. The splitting rule for 1H NMR For simple cases, the multiplicity of a signalfor a particular proton is equal to the number of equivalent vicinal protons + 1.

13. Table 13.2 (page 504) Number of equivalent Appearance Intensities of linesprotons to which H of multiplet in multipletis coupled 1 Doublet 1:1 2 Triplet 1:2:1 3 Quartet 1:3:3:1 4 Pentet 1:4:6:4:1 5 Sextet 1:5:10:10:5:1 6 Septet 1:6:15:20:15:6:1 Splitting Patterns of Common Multiplets

14. 13.8Splitting Patterns:The Ethyl Group CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet)

15. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 BrCH2CH3 Figure 13.13 (page 503) 4 lines; quartet 3 lines; triplet CH3 CH2 Chemical shift (d, ppm)

16. 13.9Splitting Patterns:The Isopropyl Group (CH3)2CHX is characterized by a doublet-septet pattern (septet at lower field than the doublet)

17. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 BrCH(CH3)2 Figure 13.15 (page 505) 2 lines; doublet 7 lines; septet CH3 CH Chemical shift (d, ppm)