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Proton NMR Spectroscopy

Proton NMR Spectroscopy

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Proton NMR Spectroscopy

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  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/2p where I = 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. 1H, 13C, 19F, 31Pall have I = 1/2 DE = g(h/2p)Bo

  4. Spin states split in the presence of B0

  5. When a nucleus aligned with a magnetic field, B0, 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 B0. A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T). Each unique probe nucleus (1H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf. All protons come into resonance between0 and 12/1,000,000 (0 – 12 ppm) of the B0.

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

  10. 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

  11. Chemical EquivalenceHow many signals in 1H NMR spectrum?

  12. Number of Equivalent Protons

  13. 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

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

  15. 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

  16. Vinyl Protons

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

  18. The d Scale

  19. Tetramethylsilane (TMS)

  20. Chemical Shift Ranges, ppm

  21. Diamagnetic AnisotropyShielding and Deshielding

  22. Deshielding in Alkenes

  23. Shielding in Alkynes

  24. Methyl t-butyl ether (MTBE)

  25. 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

  26. Integral Trace

  27. Spin-Spin Splitting

  28. The Doublet in 1H NMR

  29. Hb in 1,1,2-Tribromoethane

  30. The Triplet in 1H NMR

  31. Ha in 1,1,2-Tribromoethane

  32. 1,1,2-Tribromoethane

  33. The Quartet in 1HMR

  34. 1,1-Dichloroethane

  35. Ethyl benzene

  36. CH3CH2OCH3

  37. Equivalent Protons do not Couple

  38. Pascal’s Triangle

  39. Methyl Isopropyl Ketone

  40. 1-Nitropropane

  41. Differentiate using 1H NMR