NMR-spectroscopie: overzicht van chemische verschuivingen

The position of the NMR absorption frequency is expressed in terms of how far it is shifted from the signal of a reference compound, usually TMS.

The frequency difference is divided by the instrument operating frequency to yield the chemical shift, a dimensionless quantity.

Expressed in parts per million or ppm, chemical shifts are usually plotted on the δ scale, where the TMS signal appears at 0 ppm. 

For instance, the absorption frequency of benzene protons is 436 hertz higher than the TMS signal in a 60 megahertz spectrometer, while the difference is 2181 hertz in a 300 megahertz instrument.

The chemical shift is 7.27 ppm in both cases, making it independent of the instrument operating frequency.

Signals on the right side of the spectrum are low-frequency upfield signals arising from the shielded nuclei of electron-dense environments.

In contrast, the signals on the left are high-frequency downfield signals from deshielded nuclei in electron-poor settings.