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Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy. Principles of Molecular Spectroscopy: Electromagnetic Radiation. Electromagnetic Radiation. is propagated at the speed of light has properties of particles and waves the energy of a photon is proportional to its frequency.

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Nuclear Magnetic Resonance Spectroscopy

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  1. Nuclear Magnetic ResonanceSpectroscopy

  2. Principles of Molecular Spectroscopy:Electromagnetic Radiation

  3. Electromagnetic Radiation is propagated at the speed of light has properties of particles and waves the energy of a photon is proportional to its frequency

  4. Figure 13.1: The Electromagnetic Spectrum Shorter Wavelength (l) Longer Wavelength (l) 400 nm 750 nm Visible Light Higher Frequency (n) Lower Frequency (n) Higher Energy (E) Lower Energy (E)

  5. Figure 13.1: The Electromagnetic Spectrum Shorter Wavelength (l) Longer Wavelength (l) Ultraviolet Infrared Higher Frequency (n) Lower Frequency (n) Higher Energy (E) Lower Energy (E)

  6. Figure 13.1: The Electromagnetic Spectrum Cosmic rays g Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Energy

  7. Principles of Molecular Spectroscopy: Quantized Energy States

  8. DE = hn Electromagnetic radiation is absorbed when theenergy of photon corresponds to difference in energy between two states.

  9. What Kind of States? UV-Vis infrared microwave radiofrequency electronic vibrational rotational nuclear spin

  10. Introduction to 1H NMR Spectroscopy

  11. The nuclei that are most useful toorganic chemists are: 1H and 13C both have spin = ±1/2 1H is 99% at natural abundance 13C is 1.1% at natural abundance

  12. + + Nuclear Spin A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.

  13. + + + + The distribution of nuclear spins is random in the absence of an external magnetic field. +

  14. + + + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + H0

  15. + + + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. + H0

  16. + + Energy Differences Between Nuclear Spin States no difference in absence of magnetic field proportional to strength of external magnetic field DE ' DE increasing field strength

  17. Some important relationships in NMR The frequency of absorbedelectromagnetic radiationis proportional to the energy difference betweentwo nuclear spin stateswhich is proportional to the applied magnetic field

  18. Some important relationships in NMR Units The frequency of absorbedelectromagnetic radiationis proportional to the energy difference betweentwo nuclear spin stateswhich is proportional to the applied magnetic field Hz kJ/mol(kcal/mol) tesla (T)

  19. Some important relationships in NMR The frequency of absorbed electromagneticradiation is different for different elements, and for different isotopes of the same element. For a field strength of 4.7 T:1H absorbs radiation having a frequency of 200 MHz (200 x 106 s-1)13C absorbs radiation having a frequency of 50.4 MHz (50.4 x 106 s-1)

  20. Some important relationships in NMR The frequency of absorbed electromagneticradiation for a particular nucleus (such as 1H)depends on its molecular environment. This is why NMR is such a useful toolfor structure determination.

  21. Nuclear Shieldingand1H Chemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?"

  22. C H Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. H0

  23. C H Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. H0

  24. C H Shielding The induced field shields the nuclei (in this case, C and H) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. H0

  25. C H Chemical Shift Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. H0

  26. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 DownfieldDecreased shielding UpfieldIncreased shielding (CH3)4Si (TMS) Chemical shift (d, ppm)measured relative to TMS

  27. Cl Cl H C Cl 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 d 7.28 ppm Chemical shift (d, ppm)

  28. Effects of Molecular Structureon1H Chemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts

  29. Electronegative substituents decreasethe shielding of methyl groups CH3Fd 4.3 ppm CH3OCH3d 3.2 ppm CH3N(CH3)2d 2.2 ppm CH3CH3d 0.9 ppm CH3Si(CH3)3d 0.0 ppm

  30. Electronegative substituents decreasethe shielding of methyl groups CH3Fd 4.3 ppm least shielded H CH3OCH3d 3.2 ppm CH3N(CH3)2d 2.2 ppm CH3CH3d 0.9 ppm CH3Si(CH3)3d 0.0 ppm most shielded H

  31. Effect is cumulative CHCl3d 7.3 ppm CH2Cl2d 5.3 ppm CH3Cld 3.1 ppm

  32. H H H H H C C H H H H H Protons attached to sp2 hybridized carbonare less shielded than those attachedto sp3 hybridized carbon CH3CH3 d 7.3 ppm d 5.3 ppm d 0.9 ppm

  33. H C C C H C H H C C C C O C C H C Type of proton Chemical shift (d),ppm Type of proton Chemical shift (d),ppm 2.5 R 0.9-1.8 Ar 2.3-2.8 1.6-2.6 H 4.5-6.5 2.1-2.5 C

  34. H Cl C H Ar O H C C H H O C H NR C Type of proton Chemical shift (d),ppm Type of proton Chemical shift (d),ppm 3.1-4.1 6.5-8.5 Br 2.7-4.1 9-10 3.3-3.7 2.2-2.9

  35. H NR H OR H OAr O C HO Type of proton Chemical shift (d),ppm 1-3 0.5-5 6-8 10-13

  36. 13.6Interpreting Proton NMR Spectra

  37. Information contained in an NMRspectrum includes: 1. number of signals 2. their intensity (as measured by area under peak) 3. splitting pattern (multiplicity)

  38. Number of Signals protons that have different chemical shifts are chemically nonequivalent exist in different molecular environment

  39. N CCH2OCH3 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Figure 13.9 (page 497) OCH3 NCCH2O Chemical shift (d, ppm)

  40. Chemically equivalent protons are in identical environments have same chemical shift replacement test: replacement by some arbitrary "test group" generates same compound H3CCH2CH3 chemically equivalent

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