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Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR). for beginners. 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 ( 13 C NMR) or (rarely) other elements. Spin States.

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Nuclear Magnetic Resonance (NMR)

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  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 B’ B’ Bo

  6. Spin states in a magnetic field • Energy difference linearly depends on field strength  = magnetic moment of H (2.7927N or +14.106067x10-27J/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 60-500MHz)

  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

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