Chemical shiftIUPAC Quantity

The fractional variation of the resonance frequency of a nucleus in nuclear magnetic resonance (NMR) spectroscopy in consequence of its magnetic environment. The chemical shift of a nucleus, δ, is expressed as a ratio involving its frequency, ν cpd, relative to that of a standard, ν ref, and defined as: \[Δ \frac{\nu _{{cpd}}- \nu _{{ref}}}{\nu _{ref}}\] δ-values are normally expressed in ppm. For sup 1 /sup H and sup 13 /sup C NMR the reference signal is usually that of tetramethylsilane (TMS), strictly speaking in dilute solution in CDCl sub 3 /sub . Other references are used in the older literature and for other solvents, such as D sub 2 /sub O. Resonance lines to high frequency from the TMS signal have positive, and resonance lines to low frequency from TMS have negative, δ-values (arising from relative deshielding and shielding respectively). The archaic terms 'downfield' and 'upfield' should no longer be used. For nuclei other than sup 1 /sup H, chemical shifts are expressed either in the same manner relative to an agreed substance containing the relevant nucleus or relative to the sup 1 /sup H resonance of TMS as Ξ values, defined in the references below.