Interpreting & Explaining Carbon-13 NMR Spectroscopy (CIE A Level Chemistry)

• 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 signals on a ¹³C NMR
• 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

Chemical shift values (relative to TMS) for ¹³C NMR analysis table

Hybridisation of carbon atom	Environment of carbon atom	Example	Range of chemical shift (ppm)
sp3	alkyl	CH3–, CH2–, CH–, –C–	0 - 50
sp3	next to alkene / arene	–C–C=C, –C–Ar*	25 - 50
sp3	next to carbonyl / carboxyl	C–COR, C–CO2R	30 - 65
sp3	next to halogen	C–X	30 - 60
sp3	next to oxygen	C–O	50 - 70
sp2	alkene / arene	–C=C–, –Ar*–	110 - 160
sp2	carboxyl	R–COOH, R–COOR	160 - 185
sp2	carbonyl	R–CHO, R–CO–R	190 - 220
sp	nitrile	R–CN–	100 - 125

   * represents a benzene ring

Features of a ¹³C NMR spectrum

• ¹³C NMR spectra display sharp single signals
  • There aren’t any complicated spitting patterns as seen with ¹H NMR spectra
• The height of each signal is not proportional to the number of carbon atoms present in a single molecular environment
• CDCl₃ is used as a solvent to dissolve samples for ¹³C NMR
  • On spectra, a single solvent peak appears at 80 ppm caused by ¹³C atoms in the CDCl
  • This can be ignored when interpreting ¹³C spectra

Explaining ¹³C molecular environments

• On an organic molecule, the carbon-13 environments can be identified in a similar way to the proton environments in ¹H NMR
• For example, propanone

Identifying molecular environments in propanone

There are 2 molecular environments in propanone

• There are 2 molecular environments
  • Therefore, 2 signals will be present on its ¹³C NMR spectrum
• The carbonyl carbon will produce a ¹³C peak in the range of 190 - 220 ppm
• The 2 methyl groups will produce a ¹³C peak in the range of 30 - 65 ppm

Carbon-13 NMR spectrum for propanone

The ¹³C NMR of propanone shows 2 signals for the 2 molecular environments