Mr Cole Chemistry

Proton NMR Basics Questions

  • Questions
  • Answers
  1. CCl4 and CDCl3 are used in proton NMR.
    1. What does the symbol D represent?
    2. What are CCl4 and CDCl3 used for in proton NMR?
    3. Why is CCl4 used for this purpose rather than C6H12?
    4. CDCl3 is used sometimes and CCl4 is used other times. Explain why this is.
  2. TMS is used as a reference.
    1. Draw the skeletal formula of TMS.
    2. Explain why it makes a good reference.
  3. The following molecule was analysed using NMR spectroscopy.
    1. Predict the chemical shift range of the hydrogen atoms on carbon 1.
    2. Predict the chemical shift range of the hydrogen atoms on carbon 3.
    3. The chemical shift for the hydrogen atoms on carbon 3 was found to be 5.4. Explain the discrepancy between the expected and actual values.
    4. State the integration values for the two peaks present in the spectrum.
    5. Explain why both peaks are singlets.
    1. CCl4 and CDCl3 are used in proton NMR.
      1. What does the symbol D represent?
        Deuterium – an isotope of hydrogen that has a mass number of 2.
      2. What are CCl4 and CDCl3 used for in proton NMR?
        They are solvents.
      3. Why is CCl4 used for this purpose rather than C6H12?
        CCl­4 does not create any peaks because it does not have any hydrogen atoms.
      4. CDCl3 is used sometimes and CCl4 is used other times. Explain why this is.
        CDCl3 is polar whereas CCl4 is non-polar. You choose the appropriate solvent for the substance being analysed. If the substance is polar you use CDCl3 and if it is non-polar, you use CCl4.
    2. TMS is used as a reference.
      1. Draw the skeletal formula of TMS.
      2. Explain why it makes a good reference.
        It has 12 carbons in the same environment, so it produces one very large signal.
        There is only one environment, so there is no signal splitting.
        The signal has very low shift due to low electronegativity of Si, which means that the signal does not overlap with the other signals.
        It is very volatile so it is easy to remove.
    3. The following molecule was analysed using NMR spectroscopy.
      1. Predict the chemical shift range of the hydrogen atoms on carbon 1.
        2.1-2.6
      2. Predict the chemical shift range of the hydrogen atoms on carbon 3.
        3.7-4.1
      3. The chemical shift for the hydrogen atoms on carbon 3 was found to be 5.4. Explain the discrepancy between the expected and actual values.
        Carbon 3 is adjacent to two esters, not one, so the shift is higher.
      4. State the integration values for the two peaks present in the spectrum.
        3:2 for carbon 1 : carbon 3
      5. Explain why both peaks are singlets.
        There are no hydrogens on adjacent carbons.
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