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NMR: Theory and Equivalence

NMR: Theory and Equivalence. Nuclear Magnetic Resonance. Powerful analysis Identity Purity No authentic needed Analyze nuclei 1 H, 13 C, 31 P, etc Get information of how they are attached. Nuclear Magnetic Resonance. Nuclear Based on nuclear spin Magnetic

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NMR: Theory and Equivalence

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  1. NMR: Theory and Equivalence

  2. Nuclear Magnetic Resonance • Powerful analysis • Identity • Purity • No authentic needed • Analyze nuclei • 1H, 13C, 31P, etc • Get information of how they are attached

  3. Nuclear Magnetic Resonance • Nuclear • Based on nuclear spin • Magnetic • Large electromagnets create energy differences in nuclei • Resonance • Signal based on absorbed light in resonance with energy difference

  4. Nuclear Spin • Some nuclei have spin • 1H, 13C • No spin if nuclei have even number of protons and neutrons (12C) • Nuclei align spin with and against magnetic field

  5. External Magnetic Field • Bigger energy difference between spin states • For absorption, Ephoton = Espingap • Spin flip occurs • Consider the proton NMR spectrum of CH4

  6. NMR Spectrum • CH4 in magnet • Protons line up with/against • RF energy • Only RF energy = gap energy absorbed • Spectrum shows an absorption

  7. Magnetic Field Experienced • Not all nuclei in a molecule experience the same magnetic field, even if they are in the same magnet • Why? Shielding! • Consider CH3Cl

  8. NMR Spectrum • CH3Cl in magnet • Electronegative Cldeshield protons • Protons experience bigger external field • Only RF energy = gap energy absorbed • Spectrum shows a higher frequency absorption

  9. NMR Spectrum • Need standard: TMS (shielded) • Need relative frequency: ppm • ppm = Hz shift/ MHz magnet 60 mHz 200 MHz

  10. Other Sources of Shielding • Anisotropy • Different effect along different axis • Can have dramatic shielding/ deshielding effects

  11. Four Questions • How many signals? Equivalence • Where on spectrum? Chemical Shift • How big? Integration • Shape? Splitting (coupling)

  12. Number of Signals • Four sets of protons in different chemical environments • Four sets of equivalent protons • Leads to four distinct signals

  13. Equivalence • Experience same average magnetic field due to • Free rotation • Plane of symmetry • Axis of symmetry

  14. How many 13C signals?

  15. How many 1H signals?

  16. A more complicated problem

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