Chapter13 - C NMR Spectroscopy

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 (nuclear spin, I = 0)
As a result, C is about 400 times less sensitive than H nucleus to the NMR phenomena
Due to low abundance, we do not usually see ¹³C-¹³C coupling
Chemical shift range is normally 0 to 220 ppm
Chemical shifts 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

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

Here is the simple correlation table of ¹³C chemical shifts:

Figure of chemical shifts for different types of C

C=O indicates aldehydes and ketones
O=C-X indicates carbpxylic acids and derivatives such as esters and amides