# Edexcel A Level Chemistry:复习笔记7.7.2 Carbon-13 NMR

### Using Carbon-13 Data

• 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 the TMS) for 13C NMR Analysis Table

#### Features of a 13C NMR spectrum

• 13C NMR spectrum displays sharp single signals
• There are no complicated spitting 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

• 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
• There are 2 molecular environments
• 2 signals will be present on its 13C NMR spectrum

There are 2 molecular environments in propanone

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

#### Exam Tip

Counting the number of 13C resonances should be the first step in analysing a spectrum. For example, it is possible to differentiate the three isomers of dihydroxybenzene quickly be considering the symmetry of the molecules and therefore the number of resonances expected in their spectra.

#### Worked Example

How many chemical environments and therefore number of peaks / resonances would be in a 13C  spectra of 1,3-dihydroxybenzene?

4 chemical environments and therefore four peaks / resonances on the spectra