1. Proton NMR Spectroscopy

2. The NMR Phenomenon Most nuclei possess an intrinsic angular momentum, P. Any spinning charged particle generates a magnetic field. P = [I(I+1)] 1/2 h/2  where  spin quantum # I = 0, 1/2, 1, 3/2, 2, …

3. Which nuclei have a “spin”? If mass # and atomic # are both even, I = 0 and the nucleus has no spin. e.g. Carbon-12, Oxygen-16 For each nucleus with a spin, the # of allowed spin states can be quantized: For a nucleus with I, there are 2I + 1 allowed spin states. 1 H, 13 C, 19 F, 31 P all have I = 1/2  E =  h/2  )Bo

4. Spin states split in the presence of B 0

5. When a nucleus aligned with a magnetic field, B 0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence, NMR

6. Presence of Magnetic Field

7. NMR instruments typically have a constant Rf and a variable B 0. A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T). Each unique probe nucleus ( 1 H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf. All protons come into resonance between 0 and 12/1,000,000 (0 – 12 ppm) of the B 0.

8. Nuclei aligned with the magnetic field are lower in energy than those aligned against the field The nuclei aligned with the magnetic field can be flipped to align against it if the right amount of energy is added (DE) The amount of energy required depends on the strength of the external magnetic field

9. NMR Spectrometer Schematic diagram of a nuclear magnetic resonance spectrometer.

10. Energy Difference (  E) Between Two Different Spin States of a Nucleus With I=1/2

11. What Does an NMR Spectrum Tell You? # of chemically unique H’s in the molecule # of signals The types of H’s that are present e.g. aromatic, vinyl, aldehyde … chemical shift The number of each chemically unique H integration The H’s proximity to eachother spin-spin splitting

12. Chemical Equivalence How many signals in 1 H NMR spectrum?

13. Number of Equivalent Protons

14. Homotopic H’s –Homotopic Hydrogens Hydrogens are chemically equivalent or homotopic if replacing each one in turn by the same group would lead to an identical compound

15. Enantiotopic H’s If replacement of each of two hydrogens by some group leads to enantiomers, those hydrogens are enantiotopic

16. Diastereotopic H’s If replacement of each of two hydrogens by some group leads to diastereomers, the hydrogens are diastereotopic –Diastereotopic hydrogens have different chemical shifts and will give different signals

17. Vinyl Protons

24. Typical 1H NMR Scale is 0-10 ppm

25. The  Scale

26. Tetramethylsilane (TMS)

27. Chemical Shift Ranges, ppm

29. Diamagnetic Anisotropy Shielding and Deshielding

30. Deshielding in Alkenes

31. Shielding in Alkynes

34. Methyl t-butyl ether (MTBE)

35. Toluene at Higher Field Splitting patterns in aromatic groups can be confusing A monosubstituted aromatic ring can appear as an apparent singlet or a complex pattern of peaks

36. Integral Trace

37. Signal Splitting; the (n + 1) Rule Peak:Peak: The units into which an NMR signal is split; doublet, triplet, quartet, multiplet, etc. Signal splitting:Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens. (n + 1) rule:(n + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1 H-NMR signal is split into (n + 1) peaks.

38. Spin-Spin Splitting

39. The Doublet in 1 H NMR

40. H b in 1,1,2-Tribromoethane

41. The Triplet in 1 H NMR

42. H a in 1,1,2-Tribromoethane

43. 1,1,2-Tribromoethane

44. The Quartet in 1 HMR

45. Summary of Signal Splitting The origins of signal splitting patterns. Each arrow represents an H b nuclear spin orientation.

46. 1,1-Dichloroethane

47. Ethyl benzene

48. CH 3 CH 2 OCH 3

49. Equivalent Protons do not Couple

50. Pascal’s Triangle

51. Signal Splitting (n + 1) Problem Problem: Predict the number of 1 H-NMR signals and the splitting pattern of each.

52. Differentiate using 1 H NMR

53. Methyl Isopropyl Ketone

54. 1-Nitropropane

55. Para Nitrotoluene

56. Coupling Constants (J values)

57. Bromoethane

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

59. 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 coupling. –For H atoms three bonds apart, the coupling is called vicinal coupling.

60. Physical Basis for (n + 1) Rule Coupling that arises when H b is split by two different nonequivalent H atoms, H a and H c.

61. More Complex Splitting Patterns –Complex coupling that arises when H b is split by H a and two equivalent atoms H c.

62. More Complex Splitting Patterns –Since the angle between C-H bond determines the extent of coupling, bond rotation is a key parameter. –In molecules with free rotation about C-C sigma bonds, H atoms bonded to the same carbon in CH 3 and CH 2 groups 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. geminal coupling –This type of coupling is called geminal coupling.

63. More Complex Splitting Patterns –In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent.

64. More Complex Splitting Patterns –Tree diagram for the complex coupling seen for the three alkenyl H atoms in ethyl propenoate.

65. More Complex Splitting Patterns –Cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations. –As a result, two H atoms on a CH 2 group can be nonequivalent, leading to complex splitting.

66. More Complex Splitting Patterns –A tree diagram for the complex coupling seen for the vinyl group and the oxirane ring H atoms of 2- methyl-2-vinyloxirane.

67. More Complex Splitting Patterns Complex coupling in flexible molecules. –Coupling in molecules with unrestricted bond rotation often gives only m + n + I peaks. –That is, the number of peaks for a signal is the number of adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents. –The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp 3 hybridized C atoms.

68. More Complex Splitting Patterns –Simplification of signal splitting occurs when coupling constants are the same.

69. More Complex Splitting Patterns – Peak overlap occurs in the spectrum of 1-chloro-3- iodopropane. –H c should show 9 peaks, but because J ab and J bc are so similar, only 4 + 1 = 5 peaks are distinguishable.

70. Styrene

71. H a splitting in Styrene “Tree” Diagram

72. In the system below, Hb is split by two different sets of hydrogens : Ha and Hc –Theortically Hb could be split into a triplet of quartets (12 peaks) but this complexity is rarely seen in aliphatic systems

75. Homonuclear Decoupling

76. Allyl Bromide

78. 1-Phenyl-1,2-dihydroxyethane

80. Irradiate at  4.8