Spectroscopy

1. Spectroscopy PART II: Nuclear Magnetic Resonance Spectroscopy - Theory and 1H NMR Spectra Organic Chemistry 30B

2. Nuclear Magnetic Resonance Spectroscopy NMR Spectroscopy is the study of the interaction of electromagnetic radiation and nuclei in a magnetic field The most powerful method of molecular spectroscopy, NMR allows the determination of connectivity, stereochemistry, conformations, and even dynamics of molecular structures.

3. Energy, Wavelength, Frequency

4. NMR Spectroscopy Absorption of radio waves in the wavelength range of 10 m (frequency of 330 MHz) causes nuclear spin flips The resonance frequencies are a function of the local magnetic field which is exquisitely dependant on the molecular structure

5. Spin Nuclei have Spin Quantum Numbers

6. Spin Energies In a magnetic field different spin states have different energies

7. Spin Energies The difference in energy between the spin states is dependent on the magnitude of the applied magnetic field Bo

8. Changing Spin States

9. Resonance Frequency When a nucleus is irradiated with electromagnetic radiation at the exact frequency of precession it is in “resonance”. Energy is absorbed and the nuclear spin flips to the higher energy state. The absorption of energy is detected by the NMR spectrometer and plotted as a signal.

10. NMR Spectrometer

11. Spin Energetics The difference in energy between the spin states is dependent on the magnitude of the applied magnetic field Bo DE = h?Bo/2? h = Planck’s constant ? = Magnetogyric ratio Bo = Applied magnetic field

12. Spin Energetics The difference in energy between the spin states is dependent on the magnitude of the applied magnetic field Bo DE = h?Bo/2? But….. Molecules have electrons and the movement of electron density creates induced magnetic fields which can add to or subtract from the applied field. Therefore, nuclei in different environments will have different resonance frequencies.

13. Chemical Shift The methyl protons in propane have a resonance frequency of 400,000,360 Hz at 9.4 Tesla, while the methylene protons have a resonance frequency of 400,000,520 Hz. The exact frequencies are less important than the changes.

14. Chemical Shift Therefore, the Chemical Shift is the difference is resonance frequencies between the nucleus of interest and a reference compound, tetramethylsilane (Me4Si , TMS). The standard unit is Parts Per Million At 400 MHz, 1 ppm = 400 Hz Chemical shift of TMS defined as 0 ppm

15. Chemical Shift The standard unit is Parts Per Million At 400 MHz, 1 ppm = 400 Hz Therefore: The TMS signal at 400,000,000 Hz is defined as a Chemical Shift of 0 ppm The methyl signal in propane at 400,000,360 Hz is then 0.9 ppm The methylene signal in propane at 400,000,520 Hz is then 1.3 ppm

16. 1H NMR Spectrum

17. Chemical Shift Greater electron density creates a greater induced magnetic field to oppose the applied magnetic field. Thus the nuclei are shielded. Reduced electron density creates a lessor induced magnetic field to oppose the applied magnetic field. Thus the nuclei are deshielded.

18. Chemical Shift Proton NMR signals near 1-2 ppm are for nuclei that are shielded. Proton NMR signals greater than 2 ppm are for nuclei that are deshielded.

19. 1H NMR Spectrum

20. NMR Spectra

21. 1H NMR Spectra

22. 13C NMR Spectra

23. Equivalency of Atoms or Groups

24. Homotopic Homotopic nuclei or groups are interchangeable by an axis of rotation. Homotopic protons are equivalent so will present one signal AND will not couple to each other.

25. Homotopic Substitution Test: Two atoms or groups are homotopic if substitution of an atom or group by something else generates a structure that is identical to that generated by replacing the other atom or group.

26. Enantiotopic Enantiotopic nuclei or groups are interchangeable by a rotation and a reflection. Enantiotopic protons are equivalent by most NMR techniques so will presnt one NMR signal AND do not couple to each other.

27. Enantiotopic Substitution Test: Two atoms or groups are enantiotopic if substitution of an atom or group by something else generates the enantiomer of the compound generated by replacing the other atom or group.

28. Diastereotopic Diastereotopic nuclei are not related by any symmetry operations. Diastereotopic protons are nonequivalent so will present different NMR signals AND may couple to each other.

29. Diastereotopic Substitution Test: Two atoms or groups are diastereotopic if substitution of an atom or group by something else generates a diastereomer of the compound generated by replacing the other atom or group.

30. Topicity and Number of Signals The protons on a methyl group are Homotopic, so only one signal is seen, not three. The two methyl groups on p-xylene are Homotopic, so only one signal is seen for both. The four hydrogens on the benzene ring of p-xylene are also Homotopic, so only one signal is seen, not four.

31. Integration Integration only indicates the ratios of the number of protons Thus p-xylene has two signals in a 2 : 3 ratio

32. Chemical Shift Molecules have electrons and the movement of electron density creates induced magnetic fields which can add to or subtract from the applied field. Therefore, nuclei in different environments will have different resonance frequencies or Chemical Shifts. The Chemical Shifts in H NMR provide an indication of the type of carbon attached (sp3, sp2, sp) and the substituents (ie heteroatoms) nearby.

33. Chemical Shift

34. Chemical Shift

35. Example NMR Spectrum

36. Chemical Shift

37. Chemical Shift

38. Chemical Shift

39. Chemical Shift

40. Chemical Shift

41. Chemical Shift

42. Example NMR Spectrum

43. Example NMR Spectrum

44. Example NMR Spectrum

45. Example NMR Spectrum

46. Experimental Detail - Solvent

47. Coupling

48. Coupling

49. Coupling

50. Coupling

51. Coupling

52. Coupling

53. Coupling WHAT PROTONS COUPLE? Homotopic protons do not show coupling to each other Enantiotopic protons do not show coupling to each other Diastereotopic protons DO show coupling to each other

54. Example NMR Spectrum

55. Proton Exchange

56. Example NMR Spectrum

57. Example NMR Spectrum

58. Example NMR Spectrum

59. Unequal Coupling The magnitude of coupling is not always equal

60. Magnitude of Coupling Coupling is a through-bond process Therefore, the magnitude of coupling decreases with distance Therefore, not all couplings are equal 1J Coupling - One bond: 13C-H Alkanes (sp3 carbon) - 125 Hz Alkenes (sp2 carbon) - 150 Hz Alkynes (sp carbon) - 250 Hz

61. Magnitude of Coupling Coupling is a through-bond process 2J Coupling - Two bonds Geminal Coupling of Diasterotopic protons Up to 25 Hz

62. Magnitude of Coupling Coupling is a through-bond process 3J Coupling - Three bonds Vicinal Coupling 0 to 12 Hz depending on dihedral angle

63. Magnitude of Coupling Coupling is a through-bond process 4J or 5J Coupling - Long Range Coupling 0 to 6 Hz depending on several factors

64. Magnitude of Coupling

65. Example NMR Spectrum

66. Complex Coupling

67. NMR Spectral Problems

68. Interpreting 1H NMR Spectra Number of Signals - # of types of protons Chemical Shifts - Proton environment Integration - Ratios of numbers of protons Signal Shape - Dynamics of proton environment Signal Splitting - Number and geometry of nearby protons The goal will be determination of the complete molecular structure

69. End