The 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 in
by its frequency,
, relative to a standard,
, and defined as:
where
is the operating frequency of the
spectrometer. For
1H
and
13C
NMR the reference signal is usually that of tetramethylsilane
(
SiMe4
). Other references are used in the older literature and in other solvents, such as
D2O
. If a
resonance signal occurs at lower frequency or higher applied field than an arbitrarily selected reference signal, it is said to be
upfield, and if
resonance occurs at higher frequency or lower applied field, the signal is
downfield.
Resonance lines
upfield from
SiMe4
have positive, and
resonance lines
downfield from
SiMe4
have negative,
-values.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1096
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
doi:10.1351/goldbook.
in NMR
