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3.10 13C NMR Spectroscopy
There are a few major differences between 13C and 1H NMR, but we will focus on those.
1H is the most abundant form of hydrogen, while 13C is only a minor form of carbon.
The use of a sensitive receiver coil is required because only one in every hundred carbon atoms will resonance.
Each signal has three characteristics, chemical shift, integration, and multiplicity.
The chemical shift is important in 13C NMR.
13C signals are often not observed, which makes it easier to interpret the spectrum.
Most of the time, the integration values are useless because the pulse technique used to calculate them has a negative effect on the values.
Multiplicity is not a characteristic of typical 13C NMR experiments.
The signals are observed as singlets.
There is a special technique that suppresses 13C--1H splitting.
If we did not use this technique, the signal of each 13C atom nucleus would be split between the protons that are directly connected to it and the protons that are two or three sigma bonds removed.
The signals would overlap to create an unreadable spectrum.
The 13C signals collapse to singlets when broadband is used, which makes the spectrum more easily interpreted.
The chemical shift values range from 0 to 220 parts per million.
The number of signals in a 13C NMR spectrum represents the number of carbon atoms in different electronic environments.
The carbon atoms that are interchangeable will only produce one signal.
The compounds below are examples of this point.
Each compound has eight carbon atoms.
Each compound has the equivalent carbon atoms labeled with the same letter.
The location of each signal is dependent on shielding and de shielding effects.
There are several important types of carbon atoms.
Three carbon atoms are hybridized.
The carbon atoms were hybridized.
There are carbon atoms in this region.
The carbon atoms are protected.
This information can be used to solve the following exercise.
Determine the number of expected signals.
The compound has five carbon atoms, but we must look to see if any of them are the same.
There is symmetry and we expect three signals in the 13C spectrum.
Predict the number of signals and the location of each signal in the 13C NMR spectrum for each compound.
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