Project Details:

NMR chemical shifts: updated conventions

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.

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.

(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. The funds supplied by IUPAC for the project sufficed to obtain the necessary helium-3 gas. 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. The 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 Recommendationspublished in PureAppl. Chem.80(1), 59-84, 2008.

This article has been reprinted in:

(a) Magn. Reson. Chem. 46 (2008) 582-598 (together with an editorial letter in Magn. Reson. Chem. 46 (2008) 507)
(b) Solid State NMR 33 (2008) 41-56, and
(c) Encyclopedia of Magnetic Resonance (on-line), Editors-in-Chief R. K. Harris & R. E. Wasylishen, DOI 10.1002/9780470034590.emrstm1019

It was also the subject of an announcement in J. Magn. Reson. 191 (2008) 340-341.

Last update: 1 May 2009

Annoucement published in Chem. Int. Jul 2003

• Harris, Robin

• Becker, Edwin
• de Menezes, Sonia
• Granger, Pierre
• Hoffman, Roy
• Zilm, Kurt