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Proton NMR Spectroscopy

Proton NMR Spectroscopy. 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/2 p where I = spin quantum # I = 0, 1/2, 1, 3/2, 2, …. Which nuclei have a “spin”?.

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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 Rf.

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

  9. 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

  10. How many signals in the Proton NMR Spectrum?

  11. Shielded Protons • A naked proton will absorb at 70,459 gauss. • A shielded proton will not absorb at 70,459 gauss so the magnetic field must be increased slightly to achieve resonance.

  12. Chemical Shift • The variations in the positions of NMR absorptions, arising from electronic shielding and deshielding, are called chemical shifts. • The chemical shift (in ppm) is independent of the spectrometer used. • Most common scale is the d (delta) scale.

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

  14. The d Scale

  15. Tetramethylsilane (TMS)

  16. Electronegative Atoms • More electronegative atoms deshield more and give larger shift values. • Additional electronegative atoms cause an increase in chemical shift.

  17. Location of Electronegative Atoms • The deshielding effect of an electronegative substituent drops off rapidly with distance.

  18. Chemical Shift Ranges, ppm

  19. Diamagnetic AnisotropyShielding and Deshielding

  20. Deshielding in Alkenes

  21. Shielding in Alkynes

  22. Chemical Shift is Affected by Electron Density Around Nucleus

  23. from chemistry.msu.edu

  24. Number of Signals • Methyl tert-butyl ether has two types of protons, giving two NMR signals. • Chemically equivalent hydrogens have the same chemical shift. All the methyl groups of the tert-butyl group are equivalent, and they produce only one signal.

  25. Intensity of Signals: Integration • The amount the integral trace rises is proportional to the area of that peak. • The integration will have a trace for the tert-butyl hydrogens that is three times as large as the trace for the methyl hydrogens. The relative area for methyl and tert-butyl hydrogens is 1:3.

  26. tert-Butyl Acetoacetate • The spectrum of tert-butyl acetoacetate has only three signals. The most shielded protons are the methyl groups of the tert-butyl. The most deshielded signal is the methylene (CH2) because it is in between two carbonyl groups.

  27. Toluene at Higher Field

  28. Integral TraceHow many protons give rise to each signal?

  29. Spin-Spin Splitting

  30. The Doublet in 1H NMR

  31. Hb in 1,1,2-Tribromoethane

  32. The Triplet in 1H NMR

  33. Ha in 1,1,2-Tribromoethane

  34. 1,1,2-Tribromoethane

  35. The Quartet in 1HMR

  36. The N + 1 Rule If a proton is coupling to N equivalent protons, (on adjacent atoms) it is split into N + 1 peaks.

  37. Spin-Spin Splitting Distance • Equivalent protons do not split each other. • Protons bonded to the same carbon will split each other if they are nonequivalent. • Protons on adjacent carbons normally will split each other. • Protons separated by four or more bonds will rarely split each other.

  38. Predict Splitting

  39. 1,1-Dichloroethane

  40. Ethyl benzene

  41. Methyl Isopropyl Ketone

  42. 1-Nitropropane

  43. Hint Ethyl, propyl and isopropyl groups are common. Learn to recognize them from their splittingpatterns.

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