Worksheet: Carbon NMR Spectra

In this worksheet, we will practice interpreting ¹³C NMR spectra, using the positions and relative intensities of peaks to determine molecular structure.

Q1:

How does the range of chemical shifts in carbon-13 NMR differ from that in proton NMR?

  • ACarbon-13 and proton NMR signals span similar ranges of chemical shifts, but changes in chemical shift are harder to detect in carbon-13 NMR due to the lower signal intensity.
  • BCarbon-13 and proton NMR signals span similar ranges of chemical shifts, but the positions of carbon-13 signals are more sensitive to the effects of solvation and hydrogen bonding.
  • CCarbon-13 NMR signals span a much wider range of chemical shifts, but changes in chemical shift are harder to detect due to the lower signal intensity.
  • DCarbon-13 NMR signals span a much wider range of chemical shifts and are more sensitive to the chemical environment.
  • ECarbon-13 NMR signals span a much narrower range of chemical shifts and are less sensitive to the chemical environment.

Q2:

Why is carbon NMR harder to do than proton NMR?

  • AThe actual atoms of carbon observed (the isotope we can detect) are of lower relative abundance than those of hydrogen.
  • BThe gyromagnetic ratio (NMR sensitivity factor) is over 100 times less for carbon-13 than for hydrogen-1.
  • CThe mechanism of interaction of the isotopes of carbon (number 13) with a magnetic field is dependent on how many hydrogens are attached to it.
  • DIt has a higher atomic mass, which makes it slower to move and relax on the NMR time scale.

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