1. Nuclear Magnetic Resonance (NMR)
for beginners

2. Overview
NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules
Information can be gained from the hydrogens (proton NMR, the most common), carbons (13C NMR) or (rarely) other elements

3. Spin States All nuclei have a spin state (I )
Hydrogen nuclei have a spin of I = ±½ (like electrons)
Spin number gives number of ways a particle can be oriented in a magnetic field: 2I + 1

4. Spin States
In the absence of a magnetic field the spin states are degenerate
The “spinning” nucleus generates its own magnetic field

5. Spin States In a magnetic field the states have different energies Bo

6. Spin states in a magnetic field
Energy difference linearly depends on field strength
 = magnetic moment of H (2.7927N or x10-27J/T)

7. Spin states in a magnetic field
Even in a very large field (1-20T) the energy difference is small (~0.1cal/mol)

8. Spin states in a magnetic field
A small excess of protons will be in the lower energy state
These can be promoted to the higher state by zapping them with EM radiation of the proper wavelength
Wavelength falls in the radio/TV band (frequency of MHz)

9. Spin states in a magnetic field
Stronger magnetic field necessitates shorter wavelength (higher frequency)
After low energy protons are promoted to the higher energy state they relax back to the lower state

10. Making NMR work Not all protons absorb at the same field values
Either magnetic field strength or radio frequency must be varied
Frequency/field strength at which the proton absorbs tells something about the proton’s surroundings

11. Making NMR work

12. Sample must be spun to average out magnetic field inhomogeneity

13. NMR data collection Continuous wave data collection (CW):
Magnetic field value is varied
Intensity of emitted RF compared to RF at detector
Absorption is plotted on graph

14. NMR data collection
CW NMR of isopropanol

15. NMR data collection Pulsed Fourier transform data collection:
Short bursts of RF energy are shot at sample
Produces a decay pattern
FT done by computer produces spectrum

16. Simple FT FID and spectrum

17. More complex FT FID and spectrum

18. Even more complex FT FID

19. FT NMR Spectrum

20. Pulsed FT NMR of isopropanol

21. Chemical shift
Protons in different environments absorb at different field strengths (for the same frequency)
Different environment = different electron density around the H

22. Chemical shift positions
High field, shielded
Low field, deshielded
PPM of applied field () from reference
Reference (tetramethylsilane)

23. Chemical shift positions

24. NMR reference Tetramethylsilane ((CH3)4Si) Advantages: Makes one peak
12 equivalent H, so little is needed
Volatile, inert, soluble in organic solvents
Absorbs upfield of hydrogens in most organic compounds

25. Shielding/deshielding
Electron density affects chemical shift
Electrons generate a magnetic field opposed to the applied field
H in high electron density absorbs upfield (toward TMS, 0ppm) from H in low electron density

26. Shielding/deshielding
Effect of electronegativity: electronegative atom nearby removes electron density and causes deshielding
TMS protons are extremely shielded because Si is electropositive compared to C

27. Shielding/deshielding
Few protons absorb upfield of TMS
Alkyl groups are electron donating, so alkanes absorb around 0-2ppm ()
Hydrogens near electronegative atoms are deshielded
Absorption is around 3-4

28. Anisotropy
“Anisotropy”: any characteristic that varies with direction (asymmetric)
Applied to the shielding/deshielding characteristics of electrons in some systems

29. Anisotropy
Aromatic hydrogens are in the deshielding region of the magnetic field generated by circulating electrons

30. Typical chemical shifts

31. Spin-spin coupling
Magnetic field felt by a proton is affected by the spin states of nearby protons – either shielding or deshielding
Case 1: neighboring single protons
These H can either be the same or opposite spins – equal probability
Makes doublets of two equal peaks at both absorptions

32. NMR spectrum of dichloroacetaldehyde

33. Coupling constants Separation between peaks is the “coupling constant”
Symbol: J
Measured in Hz
It is the same for both coupled protons

34. Spin-spin coupling Case 2: Single proton next to a pair
Single proton splits the pair into a doublet
Spin state possibilities for pair:
 
Integration ratio: 1:2:1
 
  Bo
 
Equal energy

35. Spin-spin coupling Single proton is split into a triplet
Any group of n protons will split its neighbors into n + 1 peaks
Intensity follows Pascal’s triangle (Fibonacci series)

36. Spin coupling example
Chloroethane CH3CH2Cl

37. Protons on Heteroatoms
Protons on N or O often give broad uncoupled peaks of uncertain chemical shift
Protons on nitrogen are broad due to coupling with nitrogen nucleus (spin # = 1)
Chemical shift can depend on concentration
Peaks will be sharp and coupled if there is no acid or water present

38. Protons on heteroatoms
Split into doublet by NH – reciprocal splitting is not seen
Proton on nitrogen: broad due to interaction with nitrogen (spin number = 1)

39. Phenolic Protons and Concentration

40. Alcoholic protons and coupling
1H NMR spectrum of methanol at various temperatures

41. Chemical Shift Differences and Coupling
Equivalent protons do not split each other
Adjacent protons (“vicinal”) exhibit simple coupling if their chemical shifts are very different (/J >10)
Designated an “AaXx” system (“AaMmXx” for three widely separated sets)
Subscripts designate the number of protons involved

42. Chemical Shift Differences and Coupling
Sets of protons close to each other are “AaBb” or “AaBbCc”
The closer two sets are the more the peaks are distorted
AX system becoming an AB system

43. Chemical Shift Differences and Coupling

44. AX system with some distortion

45. Ternary systems
AaMmXx systems exhibit simple splitting with two coupling constants

46. Ternary Systems

47. Ternary systems

48. Chemical and magnetic equivalence

49. Chemical and magnetic equivalence

50. Chemical and magnetic equivalence
NMR spectrum of butane

51. Chemical Shift Differences and Coupling
AaBbXx systems are approximately first order (simple splitting)
AaBbCc systems are complex