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3. Chemical shifts - Positions of the resonance signals in the spectraFor the prediction of the NMR spectrum of a given molecule it is necessary to know the number of resonance signals. But it is obviously also necessary to know the position of these signals in the spectrum, that is, their chemical shift. Just as in case of the number of signals, it is also possible to make predictions about their position:
Equivalent protons give rise - due to their identical electronic environment - to chemical shifts.
If two protons have similar electronical environments, then their chemical shifts in an NMR spectrum will be .
For example, the signals of the protons in the two methyl groups -CH2-CH3 and >CH-CH3 will show chemical shifts.
The analysis of a large number of spectra showed that the signals of aliphatic methyl protons typically have a chemical shift d = 0,6 ... 1,9 ppm.
In contrast to this leads the completely different electronic environment of a carbonyl group -CHO als to a different value for the typical chemical shift. In that case d = 9,l ... l0 ppm for the proton of the aldehyde group.
An overview of the typical chemical shifts for the proton of important chemical groups is posted here.