Chapter 13: Spectroscopy

C-NMR Spectroscopy

It is useful to compare and contrast H-NMR and C-NMR as there are certain differences and similarities:

• ¹³C has only about 1.1% natural abundance (of carbon atoms)
• ¹²C does not exhibit NMR behaviour (I=0)
•  ¹³C nucleus is also a spin 1/2 nucleus
• ¹³C nucleus is about 400 times less sensitive than H nucleus to the NMR phenomena
• Due to the low abundance, we do not usually see ¹³C-¹³C coupling
• Chemical shift range is normally 0 to 220 ppm
• Chemical shifts are also measured with respect to tetramethylsilane, (CH₃)₄Si (i.e. TMS)
• Similar factors affect the chemical shifts in ¹³C as seen for H-NMR
• Long relaxation times (excited state to ground state) mean no integrations
• "Normal" ¹³C spectra are "broadband, proton decoupled" so the peaks show as single lines
• Number of peaks indicates the number of types of C

What does broadband, proton decoupled mean ?

The resonances due to 13C nuclei are split by neighbouring H atoms. These splittings would complicate the appearance of the spectra making them harder to interpret.  Therefore, in a "normal" 13C spectra, these couplings are "removed" by applying a continuous second radio frequency signal of a broad frequency range that excites all the H nuclei and cancels out the coupling patterns due to the interaction of the H with the 13C.  This means that each C is seen as a single line. Of course information is being lost by doing this, such as how many H are attached to each C.

In off-resonance decoupling the one bond C-H couplings are retained so the signal for a particular C is given by the number of attached H in accord with n+1 rule.  So, for example, a -CH₃ shows as a quartet and a -CH₂- as a triplet.

Questions

• How many lines in the peak for a CH group in an off-resonance decoupled spectra ? 
• What about a quaternary C with no H attached ? 

¹³C chemical shifts
The most significant factors affecting the chemical shifts are:

• Electronegativity of the groups attached to the C
• Hybridization of C