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Chemical Shift Scale (ppm)

The chemical shift is reported relative to a standard reference compound (TMS) and is expressed in parts per million (ppm).
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The statement of the theorem

Define the chemical shift δ\delta as the relative difference in the resonance frequency νmeas\nu_{meas} of a nucleus compared to a standard reference compound (e.g., TMS, νref\nu_{ref}), normalized by the spectrometer frequency νspec\nu_{spec} (or field strength B0B_0). The chemical shift in parts per million (ppm) is calculated by the ratio: δ (ppm)=νmeas−νrefνspec×106\delta \text{ (ppm)} = \frac{\nu_{meas} - \nu_{ref}}{\nu_{spec}} \times 10^6 where νmeas\nu_{meas} and νref\nu_{ref} are measured in Hertz (Hz), and νspec\nu_{spec} is the operating frequency in Hz. This dimensionless quantity quantifies the local electronic environment's influence on the shielding experienced by the nucleus.