1 / 31

NMR Theory and C-13 NMR

NMR Theory and C-13 NMR. 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

wilma-herrera
Download Presentation

NMR Theory and C-13 NMR

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. NMR Theory and C-13 NMR

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

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

  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

  17. Equivalence and Chiral Molecules • Homotopic • Enantiotopic • Diastereotopic

  18. How Many C-13 Signals?

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

  20. Carbon-13 Spectra • How many C-13 signals? • Which signals are further upfield? • Can the compounds be differentiated?

  21. Correlation Chart • Same number of signals • Occur in different areas of spectrum • Predictable shifts

  22. Chemical Shift Tables • Complex vs simple • Basics: • Alkane: 5-45ppm • C-Q: 40-70ppm • sp: 70-100ppm • sp2: 100-160 ppm • Carbonyl: 160-220 ppm

  23. Predict C-13 Chemical Shifts • If carbon fits two categories, choose higher shift

  24. More subtle points • Inductive effects • Additive effects • More substituted sp3 carbon is further downfield

  25. More subtle points • Carbonyls • Above 200ppm, ketone and aldehyde • Below 200ppm, O=C-Q • Explain by shielding and resonance

  26. More subtle points • Alkene vs Alkyne

  27. Two Types of Problems • Predicting spectra: given a structure, what will the C-13 NMR look like? • Useful in synthesis of known target • A little more straightforward • Interpreting spectra: given a spectrum, what is the structure of compound? • Identify an unknown; side products in synthesis • More experience needed

  28. Which Structure(s) Fit? TMS absent CDCl3

  29. Which Structure Fits?

  30. C3H8O

More Related