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Carbon (13C) NMR
13C NMR spectroscopy is best used in concert with 1H NMR for structural determination. Because 13C is only 1% abundant you do not see C-C coupling in 13C NMR and the experiment takes a long time to run.
Decoupling in 13C NMR
In order to improve the sensitivity of the 13C NMR experiment, something called decoupling is used. This means that 13C-1H coupling is not observed. This has a few consequences:
- all peaks in 13C NMR are singlets
- the height of the 13C singnal is representative of the number of hydrogens that are bound to that carbon
- CH3 are tallest
- quaternary carbons are shortest
Example: determine the number of signals you would see for each of the molecules below.
Common 13C NMR Shifts
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Example: 13C NMR
Assign the 13C NMR spectrum for 3-methyl-butane-2-one.
- the signal at ca. 215 ppm is the carbonyl carbon
- the signal at ca. 41 ppm is the C-H carbon
- the signal at ca. 28 ppm is the CH3 carbon bonded to the carbonyl
- the signal at ca. 18 ppm is the CH3 carbon (for the two CH3's)
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Example: 13C NMR
Assign the 13C NMR spectrum for para-tert-butyl-methoxybenzene.
The 13C NMR spectrum provided belongs to which of the following molecules?
The 13C NMR spectrum provided belongs to which of the following molecules?
