1. Nuclear Magnetic Resonance (NMR) Spectroscopy
Aims
What are the principles of nmr?
What is a 13C nmr spectrum?
What is chemical shift?
What information does a 13C nmr spectrum give?

2. Nuclear Magnetic Resonance (NMR) Spectroscopy
Many nuclei with odd mass numbers have the property of nuclear spin.
1H nuclei and 13C nuclei both have this property. They behave as if they were tiny bar magnets producing a very weak magnetic field.
When a strong external magnetic field is applied, the weakly magnetic nuclei in the molecules of the sample can line up in the same direction as the external field or in the opposite direction to the external field. The nuclei aligned with the field are lower in energy than those aligned against the field. This therefore produces two energy levels.

3. The stronger the external magnetic field and the stronger the magnetic field produced by the nuclei, the larger the energy difference, ∆E, between the two levels.
Slightly more than 50% of nuclei are found in the lower energy level. If the correct frequency of electromagnetic radiation is applied, some of these nuclei move up, or ‘flip’, to the higher level, absorbing the energy as they do so.
This is called resonance. The energy absorbed corresponds to radio frequency radiation.
If the magnetic field is kept constant and the radio frequency gradually increased, different nuclei will come into resonance at different frequencies, depending on the strength of the magnetic fields the different nuclei produce.

4. Carbon-13 NMR
Although only 1% of carbon atoms are 13C, modern instruments are sensitive enough to obtain a 13C spectrum. 
Not all the 13C atoms in a molecule resonate at exactly the same magnetic field strength. Carbon atoms in different environments within the molecule experience the external magnetic field differently. This is because all nuclei are shielded from the external magnetic field by the electrons that surround them.
Nuclei with more electrons around them are better shielded. The greater the electron density around a 13C atom, the smaller the external magnetic field experienced by the nucleus and the lower the frequency at which it resonates.
The nmr machine produces a graph of energy absorbed (from radio signal) (y-axis) against chemical shift/ppm (x-axis).

5. Chemical shift
The differences caused by shielding are quantified by the chemical shift, δ. Chemical shifts are measured in ppm relative to an internal standard, TMS (tetramethylsilane).
The chemical shift is related to the difference in frequency between the resonating nuclei in the compound and that of TMS. In 13C nmr, values of δ range from 0-200ppm.
In 13C nmr, carbon atoms in different environments give different chemical shift values. (See Table 3 page 250 Text Book).

6. Example: Ethanol CH3CH2OH
δ/ ppm
Type of carbon 15
-CH3 60
-CH2-OH

7. Note: In 13C nmr, the heights of the peaks are not significant.
Example: Propanone CH3COCH3
δ/ ppm
Type of carbon 30
-CH3
205
-C=O
Note: In 13C nmr, the heights of the peaks are not significant.

8. To do
Summary questions page 145
Suggest how propanal and propanone could be distinguished from their carbon-13 nmr spectra
Suggest how propan-2-ol and propanone could be distinguished from their carbon-13 nmr spectra
Predict the number of peaks in the carbon-13 nmr spectrum of
Butanone
Pentan-2-one
Pentan-3-one