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Project
Physical and
Biophysical Chemistry Division (I)
and
Inorganic Chemistry Division
(II)
Organic and Biomolecular Chemistry
Division (III)
Number: 2003-006-1-100
Title: NMR chemical shifts: updated conventions
Task Group
Chairman: Robin
K. Harris
Members: Edwin
D. Becker, Sonia
M. Cabral de Menezes, Pierre
Granger, Roy E.
Hoffman, and Kurt Zilm
Completion Date: 2008 - project completed
Objective:
To update IUPAC Recommendations 2001: NMR Nomenclature, Nuclear Spin
Properties and Conventions for Chemical Shifts [PAC
73, 1795 (2001)] by addressing several issues still to be resolved
in setting standards for chemical shifts, including temperature variation
of the NMR signals of reference compounds, the use of magic-angle
spinning for both solutions and solids, solvent effects, and magnetic
susceptibility corrections.
Description:
Nuclear magnetic resonance [NMR] has long been an invaluable technique
for determining molecular structure and for investigating a wide variety
of chemical phenomena. The cornerstone of such applications is the
chemical shift - a quantity that must be measured relative
to an agreed reference. For many years, common practice [endorsed
by IUPAC in the 1970s] was to use a separate reference for each nuclide.
However, as a result of the above-cited publication, IUPAC is now
on record as recommending that chemical shifts for all nuclides be
expressed on a unified scale relative to the proton [1H]
resonance of tetramethylsilane [TMS] in a 1 percent solution in CDCl3.
Since its publication only a year ago, this recommendation has been
well received by the international NMR community; has been widely
disseminated by republication in NMR journals and presentations at
NMR conferences, as detailed below; and has been publicized by one
of the three major NMR equipment manufacturers.
The IUPAC publication in 2001 included extensive tables
of data on NMR properties, chemical shift values of secondary references
for each nuclide, and detailed explanations of factors affecting measurements
of chemical shifts. However, in the interest of bringing that work
to closure and publishing the Recommendations, the task group was
forced to defer detailed consideration of several important matters:
-
The temperature dependence of the primary standard,
TMS in CDCl3, should be established on the basis of data
in the literature and possible new measurements.
-
Changes of solvent or concentration are known to
have major effects on chemical shifts. These effects should be evaluated
for selected systems by use of an external reference, physically
separated from the sample being studied.
-
An external reference, in turn, requires correction
for effects of the magnetic susceptibility of the sample. Methods
for measuring magnetic susceptibility and applying the correction
in a reliable manner must be analyzed.
-
The use of magic-angle spinning [MAS] can, in principle,
eliminate the need for magnetic susceptibility corrections, but
this method must be evaluated more carefully.
-
MAS is widely used for the study of solids. IUPAC
has thus far made no recommendations for referencing in solids,
a matter that should be rectified.
- Various conventions have been suggested in the literature for shielding
and/or chemical shift tensor parameters, and a preferred nomenclature
neeeds to be decided on and recommended.
This project is designed (1) to solicit the views of
a broad segment of the relevant NMR community on these and possibly
other important matters to be addressed, and (2) to bring together
a task group of experienced NMR spectroscopists to analyze the theory
and literature data, make experimental measurements where feasible,
and prepare a Recommendation that will extend and broaden IUPAC's
previous statements.
Progress:
(1 May 2005)
Following a Discussion Forum held during the International Meeting
on NMR organised by the Royal Society of Chemistry in Cambridge (England)
in the summer of 2003, each of the six task group members was assigned
one of the relevant topics and was asked to prepare a briefing paper
on it. Progress has been made with all of these, with comments sought
from all members of the task group and from some other experts in
the field. The task group concluded that for several areas - temperature
variation of chemical shifts, shape factor for magnetic susceptibility
correction, and solvent effects - new data or calculations would be
needed.
In the case of temperature variations of chemical shifts,
it was decided to undertake some new measurements with respect to
the He-3 nuclide, and one of the task group members (Dr. Roy Hoffman)
has been doing this. (See references below) Fundamental
measurements of the temperature variation of proton chemical shifts
(vs. the signal for He-3 gas) are now complete, and he has submitted
a paper (with Dr. Ted Becker) on this topic for publication in an
NMR journal (with acknowledgment of the IUPAC project). Draft papers
have also been written on shape factors for use with magnetic susceptibility
corrections (Dr. Hoffman) and on high-precision chemical shift measurements
(Prof. Pierre Granger). It is intended to use these articles as reference
material for the eventual IUPAC report on the project. A briefing
paper on shielding tensors has been sent to several experts for comment
before recommendations are drawn up and some responses have been received.
At the 46th Experimental NMR Conference, held in Providence
(Rhode Island) April 10-15, 2005, four members of the Working Party
held a discussion on the progress and future direction of the project.
A synopsis for the final report has been agreed, but several matters
need to be concluded before a full draft can be produced. It was agreed
to add a short section on the vexed question of referencing for nitrogen
chemical shifts (to be addressed by Dr. Sonia Cabral de Menezes).
The need for new data and calculations has delayed completion of the
overall project beyond the initial target date, but the quality of
the resultant IUPAC document will be markedly improved by inclusion
of these results.
(July 2005)
A first draft of the full report is now approximately two-thirds complete.
An additional section on alternative standards (especially for nitrogen)
is now planned, in addition to those determined at the Discussion
Forum mentioned above. A meeting between Professor Harris and Dr.
Becker in London took place in July 2005 and the strategy for completing
the project was discussed, with schedules set.
Published outcomes:
- 'Temperature dependence of the 1H chemical shift of tetramethylsilane
in chloroform, methanol, and dimethylsulfoxide', R.E. Hoffman and
E.D. Becker, J.
Magn. Reson.
176 (2005) 87 [doi:10.1016/j.jmr.2005.05.015]
- 'Measurement of magnetic susceptibility and calculation of shape
factor of NMR samples', R.E. Hoffman, J.
Magn. Reson.
178 (2006) 237 [doi:10.1016/j.jmr.2005.09.009]
- 'Standardization of chemical shifts of TMS and solvent signals in
NMR solvents', R.E. Hoffman, Magn.
Reson. Chem.
44 (2006) 606 [doi: 10.1002/mrc.1801]
Dec 2006 - A manuscript is being prepared
for publication in Pure Appl. Chem. A draft is submitted to
public review comments until 31 May 2007.
> see provisional recommendations
../../reports/provisional/abstract06/harris_310507.html
Jan 2008 - Project completed - IUPAC Recommendations
published in Pure
Appl. Chem.
80(1), 59-84, 2008.
Last update: 15 January 2008
<project announcement published
in Chem.
Int. Jul
2003>
Page last modified 15 January 2008.
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