¹³C NMR (Oxford AQA International A Level Chemistry)

¹³C NMR

• Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds

• Atoms with odd mass numbers usually show signals on NMR

  • For example, isotopes of atoms

  • Many of the carbon atoms on organic molecules are carbon-12

  • A small quantity of organic molecules will contain the isotope carbon-13 atoms

  • These will show peaks on a ¹³C NMR spectrum

• In ¹³C NMR, the magnetic field strengths of carbon-13 atoms in organic compounds are measured and recorded on a spectrum

• Just as in ¹H NMR, all samples are measured against a reference compound – Tetramethylsilane (TMS)

• On a ¹³C NMR spectrum, non-equivalent carbon atoms appear as peaks with different chemical shifts

¹³C NMR chemical shift data table

Type of carbon	δ / ppm
C–C	5 - 40
R-C–Cl or Br	10 - 70
R-CO–C-	20 - 50
R-C-N-	25 - 60
-C-O- (alcohols, ethers or esters)	50 - 90
-C=C-	90 - 150
R-CN	110 - 125
	110 - 160
R-CO- (esters or acids)	160 - 185
R-CO- (aldehydes or ketones)	190 - 220

Features of a ¹³C NMR spectrum

• ¹³C NMR spectra display sharp single signals

  • There are no complicated splitting patterns like ¹H NMR spectra

• The height of each signal is not proportional to the number of carbon atoms present in a single molecular environment

• Carbon atoms in different chemical environments will give resonances at different chemical shifts in a ¹³C spectrum

• As with ¹H NMR, tetramethylsilane is used as the standard reference point for ¹³C at 0 ppm

Identifying ¹³C molecular environments

• For organic molecules, the carbon-13 environments can be identified in a similar way to the proton environments in ¹H NMR

• For example, propanone:

  • There are 2 molecular environments

  • So, 2 signals will be present on its ¹³C NMR spectrum

The molecular environments of propanone

The ¹³C NMR spectrum of propanone