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13.10 Splitting Patterns: Pairs of Doublets

13.10 Splitting Patterns: Pairs of Doublets. Splitting patterns are not always symmetrical, but lean in one direction or the other. C. C. Pairs of Doublets. Consider coupling between two vicinal protons.

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13.10 Splitting Patterns: Pairs of Doublets

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  1. 13.10Splitting Patterns:Pairs of Doublets Splitting patterns are not always symmetrical, but lean in one direction or the other.

  2. C C Pairs of Doublets Consider coupling between two vicinal protons. If the protons have different chemical shifts, each will split the signal of the other into a doublet. H H

  3. C C Pairs of Doublets Let Dn be the difference in chemical shift in Hz between the two hydrogens. Let J be the coupling constant between them in Hz. H H

  4. H H C C J J AX When Dn is much larger than J the signal for each proton is a doublet, the doublet is symmetrical, and the spin system is called AX. Dn

  5. H H C C J J AM As Dn/J decreases the signal for each proton remains a doublet, but becomes skewed. The outer lines decrease while the inner lines increase, causing the doublets to "lean" toward each other. Dn

  6. H H C C J J AB When Dn and J are similar, the spin system is called AB. Skewing is quite pronounced. It is easy to mistake an AB system of two doublets for a quartet. Dn

  7. H H C C A2 When Dn = 0, the two protons have the same chemical shift and don't split each other. A single line is observed. The two doublets have collapsed to a singlet.

  8. H H OCH3 Cl H H 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Figure 13.17 (page 507) skewed doublets OCH3 Chemical shift (d, ppm)

  9. 13.11Complex Splitting Patterns Multiplets of multiplets

  10. H H H O2N m-Nitrostyrene Consider the proton shown in red. It is unequally coupled to the protons shown in blue and white. Jcis = 12 Hz; Jtrans = 16 Hz

  11. H H H O2N 16 Hz m-Nitrostyrene The signal for the proton shown in red appears as a doublet of doublets. 12 Hz 12 Hz

  12. H H H O2N Figure 13.18 (page 508) doublet doublet doublet of doublets

  13. 13.121H NMR Spectra of Alcohols What about H bonded to O?

  14. H H C O O—H The chemical shift for O—H is variable (d 0.5-5 ppm) and depends on temperature and concentration. Splitting of the O—H proton is sometimes observed, but often is not. It usually appears as a broad peak. Adding D2O converts O—H to O—D. The O—H peak disappears.

  15. 13.13NMR and Conformations

  16. NMR is "slow" Most conformational changes occur faster than NMR can detect them. An NMR spectrum is the weighted average of the conformations. For example: Cyclohexane gives a single peak for its H atoms in NMR. Half of the time a single proton is axial and half of the time it is equatorial. The observed chemical shift is half way between the axial chemical shift and the equatorial chemical shift.

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