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NMR spectroscopy

NMR spectroscopy. L.O.: State that NMR spectroscopy involves interaction of materials with low-energy radio frequency radiation. Describe the use of tetramethylsilane , TMS, as the standard for chemical shift measurement.

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NMR spectroscopy

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  1. NMR spectroscopy L.O.: State that NMR spectroscopy involves interaction of materials with low-energy radio frequency radiation. Describe the use of tetramethylsilane, TMS, as the standard for chemical shift measurement. State the need for deuterated solvents, e.g. CDCl3, when running an NMR spectrum.

  2. Some nuclei have a magnetic spin: 1H, 13C, 19F In a strong magnetic field they can line up either with the field or opposed the field.

  3. Different nuclear spins in an external magnetic field

  4. Excitation: a nucleus in its low –energy spin state can be promoted to an upper energy level. This energy is provided by a radio-frequency radiation. Relaxation: The excited nucleus will drop back to the lower energy level. Resonance: the energy provided by the radiation is the same as the energy gap between the tow spin states.

  5. The magnetic field felt by each nucleus depends on: • the applied magnetic field • Chemical environment. Electrons surrounding the nucleus generate a magnetic field.

  6. Nuclear shielding – electrons modify the magnetic field experienced by the nucleus

  7. Chemical shift, d, is a scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS at d=0.

  8. Tetramethylsilane (TMS), (CH3)4Si

  9. Carbon-13 NMR spectroscopy • L.O.: • Analyse carbon-13 NMR spectra to make predictions about the different types of carbon atoms present. • Predict the chemical shifts of carbons in a given molecule.

  10. Carbon-13 chemical shifts

  11. Electron withdrawing groups (O, CO, F, N) are less shielding. The less shielding, the higher the chemical shift.

  12. How many carbon environments?

  13. Carbon-13 NMR spectra of propan-1-ol and propan-2-ol

  14. Carbon-13 NMR spectra of propan-1-ol and propan-2-ol showing peak assignments

  15. Carbon-13 NMR spectra of (a) CH3CHBrCH3 and (b) CH3COOCH3

  16. 13C NMR TASK 1 For each of these compounds indicate the number of signals in the 13C NMR spectrum predict the approximate chemical shift of each signal using the data sheet a)methylpropene b)propene c)2-chloropropane d)Propanone e)methylamine f)ethyl propanoate g) 1,2-dibromopropane h)dimethylethyl propanoate i)but-2-ene

  17. L.O.: • Analyse carbon-13 NMR spectra to make predictions about possible structures for an unknown compound.

  18. Carbon-13 NMR spectra of isomer of C3H6O

  19. Structural isomers of the carbonyl isomers of C3H6O showing different carbon environments

  20. Carbon-13 NMR spectra of an aromatic compound with molecular formula C8 H8

  21. Structural isomers of the aromatic isomers of C8H8

  22. 13-C NMR of butane

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