What is a chemical shift in NMR
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Chemical shift - definition
Due to the dependence of the resonance frequency on the magnetic induction B0 (device-specific) the absolute specification of the frequency shift is unsuitable as a molecular parameter.
Therefore a relative quantity, the chemical shift δ, was introduced:
δ is the difference in the resonance signals of the observed nucleus in the sample νi and in a reference substance νdefault divided by the measurement frequency νO.
The quotient δ results in a number that is always in the order of 10-6 because it is in the Hz range, whereas it is given in MHz.
At 1H, the greatest difference is between 600 and 8,000 Hz, between 60 and 800 MHz.
To simplify matters, the numerical value is 106 multiplied and given δ in ppm (parts per million).
The chemical shift of the reference substance is (mostly) set to zero; the signals of all other nuclei (of one type of nucleus!) are given in ppm relative to this.
Because the chemical shift is device-independent, it was possible to create shift tables for each type of nucleus in which δ can be read off for most of the structural groups.
The approximate range of the chemical shift is e.g. for protons between 0 and 12 ppm, for carbon between 0 and 220 ppm.
Example of a 1H shift table
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