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Nmr Spectroscopy

Nmr Spectroscopy . Chem is try 330 . Objectives. Nmr basics chemical shifts relaxation times 2-Dimensional Nmr experiments COSY NOESY What kind of information do we obtain?. Nmr Basics. The signal in a nmr spectrum arises from transitions between nuclear spin states.

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Nmr Spectroscopy

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  1. Nmr Spectroscopy Chemistry 330

  2. Objectives • Nmr basics • chemical shifts • relaxation times • 2-Dimensional Nmr experiments • COSY • NOESY • What kind of information do we obtain?

  3. Nmr Basics • The signal in a nmr spectrum arises from transitions between nuclear spin states. • 1H, 13C, 31P all have a nuclear spin quantum number, I = 1/2. • The total number of spin states 2I + 1 = 2

  4. Energy Levels in the Spin 1/2 System b -1/2 a +1/2

  5. The Nmr Signal n (Hz)

  6. The Magnetisation Vector • We can represent the slight excess of spins in the  state by use of the nuclear magnetization vector, Mo z y x

  7. Shielding • The presence of the bonding electrons about the nucleus gives rise to ‘electron shields’ around the nucleus • Shielding constant 

  8. The Nmr Spectrum with Shielding s n (Hz)

  9. The Chemical Shift • Resonance frequencies are field dependent • Define a field-independent parameter - the chemical shift () • o - magnet strength in MHz

  10. The Chemical Shift Spectrum s d (ppm)

  11. J-Coupling (spin-spin splitting) • What happens when we have non-equivalent protons on adjacent C atoms?

  12. d (ppm) The Coupling Constant Jab (Hz)

  13. Ha Hb Hc The COSY Experiment • COrrelation SpectroscopY • What happens when we project the following spin system in two-dimensions? Jab Jbc

  14. d (ppm) d (ppm) The COSY Spectrum

  15. The NOESY Experiment • In the 2-D NOESY (nuclear Overhauser enhancement) experiment, we look for ‘through space’ dipolar couplings. Hb Jab > 0 Jac = 0 Ha Hc

  16. d (ppm) d (ppm) The NOESY Spectrum

  17. The Origin of Spin-Lattice Relaxation • By using a selective pulse (a 180 pulse), we can invert the populations of the nuclear spin states z y x

  18. Spin-Lattice Relaxation • The spin system attempts to re-establish the equilibrium magnetisation vector. We observe the magnitude of the magnetisation vector as a function of time after the inversion pulse is applied. (180 - n - 90 - acquire)n

  19. 1 z y x

  20. 2 z y x

  21. 3 z y x

  22. 4 z y x

  23. 5 z y x

  24. 6 z y x

  25. 7 z y x

  26. 8 z y x

  27. Magnetisation Intensity vs. Time M

  28. The Spin Lattice Relaxation Time • The time constant governing the decay process is the spin-lattice relaxation time, T1 • M = limiting value of magnetisation intensity • M = magnetisation intensity at t = 

  29. Nuclear Overhauser Enhancement (NOESY) Experiments • 2-D Nmr NOESY experiments have been used extensively in the determination of the conformations of nucleic acids, proteins, and membranes. • The presence of the cross peak in the 2-D NOESY spectrum indicates the presence of intermolecular or intramolecular dipolar interactions (representing a spatial proximity of < 0.50 nm).

  30. NOESY Spectrum for SDS/C4OH

  31. NOESY Spectrum for SDS/Bz

  32. NOESY Spectrum for DTAB/Bz

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