Chapter 19 Part IV Nuclear Magnetic Resonance Dr. Nizam M. El-Ashgar Chemistry Department

1. Chapter 19 Part IVNuclear Magnetic Resonance Dr. Nizam M. El-Ashgar Chemistry Department Islamic University of Gaza Chapter 19

2. Spin-Spin Splitting in 1H NMR Spectra • Peaks are often split into multiple peaks due to magnetic interactions between nonequivalent protons on adjacent carbons, The process is called spin-spin splitting. • The splitting is into one more peak than the number of H’s on the adjacent carbon(s), This is the “n+1 rule” • The relative intensities are in proportion of a binomial distribution given by Pascal’s Triangle • The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet, 5=pentet, 6=sextet, 7=heptet…..)

3. SPIN–SPIN COUPLING (SPLITTING): (n + 1) Rule • NMR Signals: not all appear as a single peak. • Peak: The units into which an NMR signal appears: singlet, doublet, triplet, quartet, etc. • Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens. • (n + 1) rule:If a hydrogen has nhydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks. Chapter 19

4. Origins of Signal Splitting • Signal coupling: An interaction in which the nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals. • Coupling constant (J): The separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet. • A quantitative measure of the influence of the spin-spin coupling with adjacent nuclei. Chapter 19

5. Physical Basis for (n + 1) Rule • Coupling of nuclear spins is mediated through intervening bonds. • H atoms with more than three bonds between them generally do not exhibit noticeable coupling. • For H atoms three bonds apart, the coupling is referred to as vicinal coupling. Chapter 19

6. Rules for Spin-Spin Splitting • Equivalent protons do not split each other • Protons that are farther than two carbon atoms apart do not split each other Chapter 19

7. 1H NMR—Spin-Spin Splitting • If Ha and Hb are not equivalent, splitting is observed when: • Splitting is not generally observed between protons separated by more than three  bonds. Chapter 19 7

8. The Origin of 1H NMR—Spin-Spin Splitting • Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. Let us consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs: • When placed in an applied field, (B0), the adjacent proton (CHBr2) can be aligned with () or against () B0. The likelihood of either case is about 50% (i.e., 1,000,006 vs 1,000,000). • Thus, the absorbing CH2 protons feel two slightly different magnetic fields—one slightly larger than B0, and one slightly smaller than B0. • Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet, where the two peaks of the doublet have equal intensity. Chapter 19 8

9. Origins of Signal Splitting Ha and Hb are non-equivalent Chapter 19

10. Chapter 19

11. Origins of Signal Splitting • When the chemical shift of one nucleus is influenced by the spin of another, the two are said to be coupled. • Consider nonequivalent hydrogens Ha and Hb on adjacent carbons. • The chemical shift of Ha is influenced by whether the spin of Hb is aligned with or against the applied field Chapter 19

12. Chapter 19

13. Chapter 19

14. Chapter 19 14

15. Spin-Spin Splitting • Non equivalent protons on adjacent carbons always interact each other. • Equivalent protons do not not split each other. CH3 – CO - CH3Do not split CH3 – CH2 - Cl Split each other Chapter 19

16. Spin-Spin Splitting Chapter 19

17. Spin-Spin Splitting Chapter 19

18. Spin-Spin Splitting Chapter 19

19. Spin-Spin Splitting • If a signal is split by N equivalent protons, it is split into N + 1 peaks. Chapter 19

20. Pascal’s Triangle: • As illustrated by the highlighted entries, each entry is the sum of the values immediately above it to the left and the right .

21. Coupling Constants (J) Distance between the peaks of multiplet measured in Hz (usually 0-18) called coupling constant. J is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled. Not dependent on strength of the external field. Gives info on type of H. Multiplets with the same coupling constants may come from adjacent groups of protons that split each other. Structural features. Chapter 19

22. An important factor in vicinal coupling is the angle a between the C-H sigma bonds and whether or not it is fixed. • Coupling is a maximum when a is 0° and 180°; it is a minimum when a is 90° Chapter 19

23. Typical coupling constant Chapter 19

24. Chapter 19

25. Spin Decoupling • It’s a powerful tool for determining 1. The connectivity of the protons. 2. Assigning proton peaks • Irradiation of one proton in a spin coupled system removes its coupling effect on the neighboring protons to which it had coupled. Chapter 19

26. Chapter 19

27. Ethyl acetate (HW) Chapter 19

28. Chapter 19

29. Ethyl Bromide Chapter 19

30. 1H-NMR spectrum of 1,1-dichloroethane Chapter 19

31. Expansion of Spectrum (HW) • Because splitting patterns from spectra taken at 300 MHz and higher are often difficult to see, it is common to retrace and expand certain signals. • 1H-NMR spectrum of 3-pentanone; expansion more clearly shows the triplet/quartet Chapter 19

32. Spectra Involving Chemical Exchange Processes Pure dry liquid ethanol. Ethanol containing a very small amount of HCl. Note: in this case there is no change in chemical shift(s), only in splitting pattern(s). Chapter 19

33. Spectra Involving Chemical Exchange Processes The observed signal is the result of the weighted average of the nucleus in its different magnetic environments. Fast exchanges show up as sharp signals. Exchanges on the NMR timescale (“intermediate”) show up as broad signals. (presence of acid/base catalyst, temperature, nature of the solvent, etc.) Slow exchanges will show two separate lines. Chapter 19

34. O-H and N-H Signals • Chemical shift depends on concentration. • Hydrogen bonding in concentrated solutions deshield the protons, so signal is around  3.5 for N-H and  4.5 for O-H. • Proton exchanges between the molecules broaden the peak. Chapter 19

35. Carboxylic Acid Proton, 10+ Chapter 19 =>

36. Splitting of Hydroxyl Proton • Ultrapure samples of ethanol show splitting • Ethanol with a small amount of acidic or basic impurities will not show splitting Chapter 19

37. N-H Proton • Moderate rate of proton transfer • Peak may be broad Chapter 19

38. 1H NMR—Spin-Spin Splitting Whenever two (or three) different sets of adjacent protons are not equivalent to each other, use the n + 1 rule to determine the splitting pattern only if the coupling constants (J) are identical: Jab = Jbc Chapter 19 38

39. 1H NMR—Structure Determination

40. Chapter 19

42. Signal Splitting (n + 1) example Chapter 19

43. More Complex Splitting Patterns • Thus far, we have observed spin-spin coupling with only one other nonequivalent set of H atoms. • More complex splittings arise when a set of H atoms couples to more than one set H atoms. • A tree diagram shows that when Hb is adjacent to nonequivalent Ha on one side and Hc on the other, coupling gives rise to a doublet of doublets. Chapter 19

44. If Hc is a set of two equivalent H, then the observed splitting is a doublet of triplets. Chapter 19

45. Chapter 19

46. Because the angle between C-H bond determines the extent of coupling, bond rotation is a factor. • In molecules with relatively free rotation about C-C sigma bonds, H atoms bonded to the same carbon in CH3 and CH2 groups generally are equivalent. • If there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent. • Nonequivalent H on the same carbon will couple and cause signal splitting, this type of coupling is called geminal coupling. Chapter 19

47. In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent. Chapter 19

48. A tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate. Chapter 19

49. Cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations • As a result, two H atoms on a CH2 group can be nonequivalent, leading to complex splitting. Chapter 19

50. A tree diagram for the complex coupling in 2-methyl-2-vinyloxirane Chapter 19