Nuclear Magnetic Resonance Spectroscopy

1. B.V.V. Sangha’s Shri S. R. Kanthi College of Arts, Commerce & Science, Mudhol Nuclear Magnetic Resonance Spectroscopy Dr Vinayak S Hegde P.G. Department of Chemistry Basaveshwar Science College, Bagakot

2. 1nm 102 103 104 105 106 107 10 NMR Spectroscopy Where is it? (the wave) X-ray UV/VIS Infrared Microwave Radio Frequency (the transition) electronic Vibration Rotation Nuclear (spectrometer) X-ray UV/VIS Infrared/Raman NMR Fluorescence

3. The entire electromagnetic spectrum is used by chemists: Frequency, nin Hz ~1019 ~1017 ~1015 ~1013 ~1010 ~105 Wavelength, l ~.0001 nm ~0.01 nm 10 nm 1000 nm 0.01 cm 100 m Energy (kcal/mol) > 300 300-30 300-30 ~10-4 ~10-6 g-rays X-rays UV IR Microwave Radio Visible

4. Introduction • NMR is the most powerful tool available for organic structure determination. • It is used to study a wide variety of nuclei: • 1H • 13C • 15N • 19F • 31P

5. Before using NMR What are N, M, and R ? Properties of the Nucleus Nuclear spin Nuclear magnetic moments The Nucleus in a Magnetic Field Precession and the Larmor frequency Nuclear Zeeman effect & Boltzmann distribution When the Nucleus Meet the right Magnet and radio wave Nuclear Magnetic Resonance

6. Edward M. Purcell 1912-1997 Kurt Wuthrich 1938- Richard R. Ernst 1933- CW NMR 40MHz 1960 Felix Bloch 1905-1983

7. 800 MHz

8. NMR Periodic Table • NMR “active” Nuclear Spin (I) = ½: • 1H, 13C, 15N, 19F, 31P • biological and chemical relevance • Odd atomic mass • I = +½ & -½ • NMR “inactive” Nuclear Spin (I) = 0: • 12C, 16O • Even atomic mass & number • Quadrupole Nuclei Nuclear Spin (I) > ½: • 14N, 2H, 10B • Even atomic mass & odd number • I = +1, 0 & -1

9. Magnetic Moment (m) • spinning charged nucleus creates a magnetic field Magnetic moment Similar to magnetic field created by electric current flowing in a coil • “Right Hand Rule” • determines the direction of the magnetic field around a current-carrying wire and vice-versa

10. Gyromagnetic ratio (g) • related to the relative sensitive of the NMR signal • magnetic moment (m) is created along axis of the nuclear spin • where: • p – angular momentum • g – gyromagnetic ratio (different value for each type of nucleus) • magnetic moment is quantized (m) • m = I, I-1, I-2, …, -I • for common nuclei of interest: • m = +½ & -½

11. Gyromagnetic ratio (g) • magnetic moment is quantized (m) • m = I, I-1, I-2, …, -I • for common nuclei of interest: • m = +½ & -½

12. Bo Magnetic alignment = g h / 4p Add a strong external field (Bo) and the nuclear magnetic moment: aligns with (low energy) against (high-energy) In the absence of external field, each nuclei is energetically degenerate

13. Bo Magnetic alignment = g h / 4p Add a strong external field (Bo) and the nuclear magnetic moment: aligns with (low energy) against (high-energy) In the absence of external field, each nuclei is energetically degenerate

14. NUCLEAR SPIN The nuclei of some atoms have a property called “SPIN”. ….. we don’t know if they actually do spin! Each spin-active nucleus has a number of spins defined by its spin quantum number, I. NMR Discovered by F. Bloch and E. Purcell

15. => Nuclear Spin • A nucleus with an odd atomic number or an odd mass number has a nuclear spin. • The spinning charged nucleus generates a magnetic field. Chapter 13

16. External Magnetic Field When placed in an external field, spinning protons act like bar magnets. => Chapter 13

17. Nuclear Spin Energy Levels N -1/2 unaligned In a strong magnetic field (Bo) the two spin states differ in energy. +1/2 aligned Bo S

18. Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. => Chapter 13

19. THE ENERGY SEPARATION DEPENDS ON Bo - 1/2 = kBo = hn DE degenerate at Bo = 0 + 1/2 Bo increasing magnetic field strength

20. E and Magnet Strength • Energy difference is proportional to the magnetic field strength. • E = h = h B02 • Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for H). • In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. • Low energy, radio frequency. => Chapter 13

21. What is NMR? NMR – Nuclear Magnetic Resonance is a branch of spectroscopy that deals with the phenomenon found in assemblies of large number of nuclei of atoms that possess both “magnetic moments” and “angular momentum” is subjected to external magnetic field. Resonance – Implies that we are in tune with a natural frequency of the nuclear magnetic system in the magnetic field.

22. PRINCIPLE: When energy in the form of Radiofrequency is applied the nuclei are said to be in resonance, and the energy they emit when flipping from the high to the low energy state can be measured and the obtained absorption of energy occurs and a NMR signal is recorded . The enery (ΔE= hv) required to resonate (or flip α to β spin states) each type of protons are note same, hence different kinds of protons give different. Applied frequency = Precessional frequency frequency of the incoming radiation that will cause a transition √ ὰ Bo strength of the magnetic field The Larmor Equation!!!

23. Magnetic Shielding • If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. • But protons are surrounded by electrons that shield them from the external field. • Circulating electrons create an induced magnetic field that opposes the external magnetic field. => Chapter 13

24. => Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. Chapter 13

25. => Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts. Chapter 13

26. Reference Std. Tetramethylsilane(TMS) • Chemically inert • Magnetically Isotopic • Sharp single only one peak on NMR spectrum • Molecule must be soluble in most of the organic solvent • Must be relatively volatile • High electronic density of H in TMS. (Almost all the H peaks of organic compounds appear on the left of the TMS peak.) • Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.

27. CHEMICAL SHIFT: () • The separation between the frequencies for the same nucleus in different chemical environments is known as chemical shift. OR • The difference in absorption of a particular proton (nuclei) from the absorption position of reference protons called chemical shift. • √r= Frequency of the reference • √s= Frequency of the sample • √o= The frequency at which the Spectrometer operates √sample– √ref X 106  ppm √o

28. Delta Scale

29. NMR Spectrum • A Spectrum of Absorption of Radiation Vs. Applied Magnetic Strength is called as NMR Spectrum. • Number of Signal-shows how many different kinds of protons are present. • Intensity of the Signal- Proportional to the number of protons of each kinds. • Location of the Signals- shows how shielded or deshielded the proton is. • Signal splitting- shows the number of protons on adjacent atoms.

30. Magnetic Shielding: • If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. • But protons are surrounded by electrons that shield them from the external field. • Circulating electrons create an induced magnetic field that opposes the external magnetic field. => Chapter 13

31. Protons in a Molecule • Depending on their chemical environment, protons in a molecule are shielded by different amounts. Chapter 13

32. FACTORS INFLUENCING CHEMICAL SHIFT • Both 1H and 13C Chemical shifts are related to the following major factors: • Depends on Hydrogen bonding • Depends on adjacent group • Depends on carbon group attached • Depends on hybridization • Depends on anisotropy

33. Location of Signals • More electronegative atoms deshield more and give larger shift values. • Effect decreases with distance. • Additional electronegative atoms cause increase in chemical shift. => Chapter 13

34. Typical Values

35. Diamagnetic shielding: Aromatic Protons, 7-8 • In a magnetic field, the six  electrons in benzene circulate around the ring creating a ring current. • The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. • The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield

36. Vinyl Protons, 5-6 1H NMR—Chemical Shift Values 4.5-6 ppm • In a magnetic field, the loosely held  electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons. • The protons now feel a stronger magnetic field, and require a higher frequency for resonance. Thus the protons are deshielded and the absorption is downfield.

37. Acetylenic Protons, 2.5 • In a magnetic field, the  electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B0). • Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield.

38. POPULATION AND SIGNAL STRENGTH The strength of the NMR signal depends on the Population Difference of the two spin states Radiation induces both upward and downward transitions. induced emission resonance For a net positive signal there must be an excess of spins in the lower state. excess population Saturation = equal populations = no signal

39. A Simplified 60 MHzNMR Spectrometer hn RF (60 MHz) Oscillator RF Detector absorption signal Recorder Transmitter Receiver MAGNET MAGNET ~ 1.41 Tesla (+/-) a few ppm N S Probe

40. Instrumentation: • Main features of a basic NMR include: • A radio transmitter coil that produces a short powerful pulse of radio waves • A powerful magnet that produces strong magnetic fields • The sample is placed in a glass tube that spins so the test material is subject to uniform magnetic field. • A radio receiver coil that detects radio frequencies emitted as nuclei relax to a lower energy level • A computer that analyses and record the data

41. The NMR Spectrometer:

42. 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 13

43. Carboxylic Acid Proton, 10+

44. Number of Signals Equivalent hydrogens have the same chemical shift.

45. => Intensity of Signals The area under each peak is proportional to the number of protons. Shown by integral trace.

46. => How Many Hydrogens? When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens.

47. Spin-Spin Splitting • It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible. • 13C will magnetically couple with attached protons and adjacent protons. • These complex splitting patterns are difficult to interpret. => Chapter 13

48. 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. => Chapter 13

49. Doublet: 1 Adjacent Proton

50. Triplet: 2 Adjacent Protons