Return to Contents Chapter 13: Spectroscopy Ch 13 contents

C-NMR Spectroscopy

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

The general implications of these points are that 13C-NMR spectra take longer to acquire than H-NMR, though they tend to look simpler. Accidental overlap of peaks is much less common than for H-NMR which makes it easier to determine how many types of C are present.

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


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

A simple correlation table of 13C chemical shifts is shown below.

Figure of chemical shifts for different types of C

 


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  Dr. Ian Hunt, Department of Chemistry, University of Calgary