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Chemical Shift
The chemical shift in absolute terms is defined by the frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm. The scale is made more manageable by expressing it in parts per million (ppm) and is indepedent of the spectrometer frequency. 

equation defining chemical shift

It is often convienient to describe the relative positions of the resonances in an NMR spectrum. For example, a peak at a chemical shift, δ, of 10 ppm is said to be downfield or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is upfield or shielded with respect to the peak at 10 ppm.  The terms shielded and deshielded will be explained later.

upfield and downfield regions of nmr spectra

Typically for a field strength of 4.7 T the resonance frequency of a proton will occur around 200MHz and for a carbon, around 50.4MHz. The reference compound is the same for both, tetramethysilane (Si(CH3)4).


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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary