Chemical shift, d, is the term given to the positions of the resonances in an NMR spectrum. It is cited in parts-per-million, or ppm, of frequency. The position of a resonance, in absolute frequency terms, varies with the exact field strength of the magnet used. Thus, peak positions would be instrument-dependent if they were expressed in Hz. To avoid this potentially confusing situation, the frequency of a resonance (in Hz) is divided by the resonant frequency of that nuclide. Furthermore, it is calculated as a shift, or difference, from the frequency of a chemical-shift reference compound. Thus, if a 1H peak is located 600 Hz away from the reference, and if the 1H resonant frequency on that spectrometer is 300 MHz (300,000,000 Hz), the chemical shift of the 1H peak in question is 2 ppm (600 Hz/300,000,000 Hz). On another instrument, whose 1H frequency is 600 MHz, the same 1H peak would be found 1200 Hz from the reference (1200 Hz/600,000,000 Hz).
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