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NMR

NMR. John Williams Professor of Chemistry Rhode Island College. NMR: OBSERVING AN ABSORPTION. INTERACTIONS OF RADIATION AND MATTER. CONSERVATIVE DISRUPTS THE RADIATION, NOT THE MATTER X-RAY, UV-VIS, IR, MICRO, RF REFLECTION, REFRACTION, DIFFRACTION, ABSORPTION DESTRUCTIVE

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NMR

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  1. NMR John Williams Professor of Chemistry Rhode Island College

  2. NMR: OBSERVING AN ABSORPTION

  3. INTERACTIONS OF RADIATION AND MATTER • CONSERVATIVE DISRUPTS THE RADIATION, NOT THE MATTERX-RAY, UV-VIS, IR, MICRO, RF REFLECTION, REFRACTION, DIFFRACTION, ABSORPTION • DESTRUCTIVE CONVERTS RADIATION AND MATTER INTO PARTICLES FAR-UV, X-RAY, NUCLEAR IONIZATION

  4. MAGNETIC FIELDS INTERACT • ODD SPIN NUCLEI ARE MAGNETIC • EARTH’S MAGNETIC FIELD NO EFFECT • ~20K GAUSE MAGNET SEPARATES SPIN ‘UP” AND “DOWN” ENERGY STATES

  5. THE PLANK CONDITION IS MET FOR RF E = h for radio frequencies Absorption of rf radiation results in a momentary reorientation of magnetic nuclei. Each nucleus absorbs at a frequency determined by its chemical “environment.” These events are recorded as an NMR spectrum.

  6. THE RESONANCE EVENT

  7. THE RESONANCE EVENT

  8. THE RESONANCE EVENT

  9. THE RESONANCE EVENT

  10. THE RESONANCE EVENT

  11. THE RESONANCE EVENT

  12. NMR Spectrum Intensity vs Chemical Shift Intensity ~ Number of protons with a given chemical shift Chemical shift = f (structure) Structure = Chemical “Environment” Electronic Stereochemical Nearby H atoms

  13. The Chemical Shift: “Naked” Proton

  14. Magnetic Field at Hydrogen for Resonance

  15. Magnetic Field at Hydrogen Bonded to an sp3 Carbon

  16. Changes Field at the Hydrogen

  17. Resonance for Proton Shielded by Electrons

  18. Proton NMR and TMS

  19. The 0.00 is the defined Chemical Shiftfor TMS • TMS is an internal standard that is added to solutions. • ~95% of all protons in organic molecules have chemical shifts downfield if TMS

  20. Neopentane has a chemical shift of 0.89 downfield of TMS

  21. Benzene has a chemical shift7.26 downfield from TMS

  22. “Ring Current” from pi electron circulation is desheilding.

  23. Electronegative atoms causedownfield chemical shifts

  24. Fig. 13.7

  25. Electronegative atoms and pi ring current together

  26. Dipole Deshielding

  27. Ring Current Deshilding

  28. Fig. 13.8

  29. Dipolar sp-1s and sp-sp3Bonds

  30. Pi Electron Circulation

  31. Fig. 13.9

  32. Spin-Spin Splitting

  33. Spin-Spin Splitting

  34. Coupling Constants • The separation between split peaks is the coupling constant. • The coupling constant indicates the strength of the splitting interaction. • It depends on distance and stereochemistry.

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