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Outcomes

Outcomes. State that NMR spectroscopy involves interaction of materials with low-energy radio-frequency radiation. State the need for deuterated solvents. Describe the use of TMS (tetramethylsilane) as the standard for chemical shift measurements.

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Outcomes

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  1. Outcomes • State that NMR spectroscopy involves interaction of materials with low-energy radio-frequency radiation. • State the need for deuterated solvents. • Describe the use of TMS (tetramethylsilane) as the standard for chemical shift measurements. • Analyse carbon-13 NMR spectra to make predictions about the different types of carbon atoms present. • Predict the chemical shifts of carbons within a given molecule.

  2. 12C atoms do not have nuclear spin, but 1.1% of carbon atoms are 13C and these do have nuclear spin and so produce NMR spectra. NMR spectra give a lot of valuable information about the chemical environment of C atoms (e.g. the difference between C atoms in C=O, C-N, C≡N, C-C, C=C, etc.). Samples carried out in solution – to prevent the C/H atoms in the solvent giving a signal Deuterium (isotope of H) is used as it has an even number of nucleons so will produce no NMR signal. Carbon-13 NMR Spectroscopy

  3. Energy S S D E N N Magnetic field NMR Spectrometry: Spin states – this makes them behave like tiny magnets. – this makes them behave like tiny magnets. They will match or oppose an external field. Neutrons & Protons possess spin Nuclei that oppose the magnetic field have a higher energy level than those aligned with the field. Both Protons Proton B Proton A Proton B Proton A Radio waves are required to ‘flip’ the nuclei so that all nuclei are no longer aligned with the magnetic field.

  4. detector coil can be the same coil! RF signal coil NMR Spectrometry: Excitation & Relaxation A low-energy nuclei (aligned with the applied field) will jump to a high energy spin state (opposing the applied field) when given a pulse of RF. (Excitation) Against field With field Induced signal When the magnetic field is removed, the nuclei revert back to their original state releasing the energy that was just given to them in the form of radiation. (Relaxation) The cycle of excitation and relaxation of the nucleus is called resonance – hence the name NMR...nuclear magnetic resonance.

  5. Chemical shift • The chemical shift (δ) is measured relative to TMS • TMS is chemically unreactive and is very volatile so is easily removed from the sample after running the NMR spectrum. TMS = 0ppm δ (ppm)

  6. The carbon in the CH3 group is attached to 3 hydrogens and a carbon. The carbon in the CH2 group is attached to 2 hydrogens, a carbon and an oxygen. But which is which?

  7. A table of typical chemical shifts in C-13 NMR spectra

  8. CH3 CH2 The external magnetic field experienced by the carbon nuclei is affected by the electronegativity of the atoms attached to them. The effect of this is that the chemical shift of the carbon increases if you attach an atom like oxygen to it.

  9. Predict the number of signals 3 - methylpentane

  10. Predict the number of signals Cyclohexane

  11. Predict the number of signals Butylamine

  12. Predict the number of signals 2 - chlorobutane

  13. Predict the number of signals 4-pentenoic acid

  14. Predict the number of signals methyl butanoate

  15. Predict the number of signals Pentan-3-one

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