Chemical Shift
The position of an NMR signal relative to a reference, expressed in parts per million of the operating frequency.
Formula / equation
δ = (ν_sample − ν_ref) × 10⁶ / ν_spectrometer
Unit
ppm
In depth
Electrons around a nucleus shield it from the applied field, so electron-withdrawing neighbours deshield it and shift the signal downfield to higher delta. Because the scale is expressed as a fraction of the operating frequency, shifts are the same on any instrument.
Examples in the real world
A methyl group appears near 0.9 ppm while an aldehyde proton appears near 9.8 ppm.