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Chapter 13 Nuclear Magnetic Resonance Spectroscopy

Chapter 13 Nuclear Magnetic Resonance Spectroscopy. Infrared Spectroscopy. Mass Spectrometry. Nuclear Magnetic Resonance Spectroscopy. NMR Spectroscopy. Maps the carbon-hydrogen framework Probes molecular structure in greater detail than IR or MS

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Chapter 13 Nuclear Magnetic Resonance Spectroscopy

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  1. Chapter 13Nuclear Magnetic Resonance Spectroscopy

  2. Infrared Spectroscopy Mass Spectrometry

  3. Nuclear Magnetic Resonance Spectroscopy

  4. NMR Spectroscopy • Maps the carbon-hydrogen framework • Probes molecular structure in greater detail than IR or MS • A complete molecular structure can oftentimes be determined by NMR alone • NMR has revolutionized organic chemistry since its introduction in the 1950’s • Before the advent of NMR, structure elucidation may have taken months (or years!) 13.1 An Overview of Proton NMR Spectroscopy

  5. NMR Machine

  6. Sample + CDCl3 & (CH3)4Si

  7. The NMR Spectrometer 13.11 The NMR Spectrometer

  8. NMR Spectroscopy • Nuclei with an odd # of protons and/or an odd # of neutrons exhibit magnetic properties • called “spin” • Nuclei with even numbers of BOTH P’s and N’s = no magnetic phenomena • 1H and 13C are the most important of the spin active nuclei

  9. Nuclear spin causes the nuclei to behave like a tiny magnets

  10. Nuclear Spin • 1H (and 13C) nuclei can have one of two spin states • quantum numbers +½ or -½

  11. Protons in a Magnetic Field 13.2 Physical Basis of NMR Spectroscopy

  12. Protons in a Magnetic Field • This absorption isnuclear magnetic resonance andis detected by an NMR spectrometer

  13. The 1H NMR Spectrum • Absorptions detect the protons in the molecule • Separate resonance for each chemically non-equivalent set of nuclei • The size (area) of a peak is proportional to the number of contributing protons

  14. Chemical Shift • The local magnetic field (Bp) “sensed” by a proton is different than the applied magnetic field (B0) • This is due to the electrons around the proton which oppose the external field • Bp = B0 – Be • The reduction of the local field is called shielding

  15. Chemical Shift • Hence, the electronegativity of nearby atoms will affect the shielding around a proton • Electronegative neighbors = deshielding • H in question “feels” external magnetic field more • Electropositive neighbors = increase in shielding • H in question “feels” external magnetic field less

  16. Chemical Shift

  17. Chemical Shift and Structure • Electronegativities of nearby groups is one of the most important factors in chemical shift

  18. Chemical Shift • The amount of alkyl substitution will also affect chemical shift

  19. Chemical Shift • A proton near more than one functional group will be affected by both

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