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,
,
is expressed as a ratio involving its frequency,
,
relative to that of a standard,
, and defined as:
-values are normally expressed in
.
For
1H and
13C
NMR the reference signal is usually that of tetramethylsilane (
TMS), strictly speaking in
dilute solution in
CDCl3. Other references are used in the older literature and for other
solvents, such as
D2O.
Resonance lines to high frequency from the
TMS signal have positive, and
resonance lines to low frequency from
TMS have
negative,
-values (arising from relative
deshielding and
shielding respectively). The archaic terms
'
downfield' and '
upfield' 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
nucleus or relative to the
1H
resonance of
TMS as
values, defined in the references below.
Source:
PAC, 2001, 73, 1795
(NMR nomenclature. Nuclear spin properties and conventions for chemical shifts (IUPAC Recommendations 2001)) on page 1807
PAC, 2008, 80, 59
(Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008)) on page 61
