chemical shift, \(\delta\)
The fractional variation of the @R05326@ frequency of a @N04256@ in nuclear magnetic @R05326@ (NMR) @S05848@ in consequence of its magnetic environment. The chemical shift of a @N04256@, \(\delta \), is expressed as a ratio involving its frequency, \(\nu _{\mathrm{cpd}}\), relative to that of a standard, \(\nu _{\mathrm{ref}}\), and defined as: \[\delta =\frac{\nu _{\mathrm{cpd}}- \nu _{\mathrm{ref}}}{\nu _{ref}}\] \(\delta \)-values are normally expressed in \(\mathrm{ppm}\). For 1H and 13C NMR the reference signal is usually that of tetramethylsilane (TMS), strictly speaking in @D01739@ in CDCl3. Other references are used in the older literature and for other solvents, such as D2O. @R05326@ lines to high frequency from the TMS signal have positive, and @R05326@ lines to low frequency from TMS have negative, \(\delta \)-values (arising from relative @S05644-1@ and @S05644-2@ respectively). The archaic terms '@D01856@' and '@U06575@' should no longer be used. For nuclei other than 1H, chemical shifts are expressed either in the same manner relative to an agreed substance containing the relevant @N04256@ or relative to the 1H @R05326@ of TMS as \(\Xi \) values, defined in the references below.
Green Book, 3rd ed., p. 29 [Terms] [Book]
PAC, 2001, 73, 1795. (NMR nomenclature. Nuclear spin properties and conventions for chemical shifts(IUPAC Recommendations 2001)) on page 1807 [Terms] [Paper]
PAC, 2008, 80, 59. (Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008)) on page 61 [Terms] [Paper]