Department of Chemistry, University of Alberta        March  2007
NMR News 2007-01
News and tips from the NMR support group for users of the Varian NMR systems in the Department

There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.

Other content of this NMR News is no longer meaningful and has been removed May 2010.



changes to the on-line reservation system: holidays


FAQ 2007-01.1: what are the significant figures in chemical shift and coupling constant measurements ?


Changes to the on-line reservation system: holidays
Since its inception in May 1999, the on-line reservation system's booking rules treated holidays in the same way as any other  working day. This has been changed on all spectrometers effective immediately.

Official holidays and days the University is closed (for example between Christmas and New Year) will now be treated as weekend days, i.e. the weekend reservation rules apply. This gives the smaller number of users requiring access to spectrometers on such days much longer instrument usage. The reservation pages indicate right under the title when "holiday | U of A closed" rules apply.

FAQ 2007-01.1: what are the significant figures in chemical shift and coupling constant measurements?
NMR experiments, like many other scientific experiments, have errors in measurements associated with them. The most common parameters that are reported are chemical shifts and coupling constants. Many factors influence how precise these values can be extracted from a spectrum. The most obvious are limitations based on digital resolution, others are more subtle and less obvious to the casual user of NMR instrumentation:

Temperature control of the sample

Chemical shifts are, strictly speaking, temperature-dependent although the effect is quite small for organic solvents (exceptions are D2O and CD3OD). Therefore, how well the sample temperature can be controlled and how precisely this can be reproduced on another spectrometer, or even on the same spectrometer if the experiment is repeated, affects the significant figures.
Spin lock in 2D techniques such as GTOCSY, TROESY: Although spin-locks come in various "flavors", they are in essence a large number of radio frequency pulses over a period of time which is often called the mixing time. The molecules in the sample absorb the energy created by the spin-lock and the end effect is sample heating that is not visible on the temperature status display of the spectrometer but visible in comparing e.g. chemical shifts in a GCOSY and a GTOCSY (even when recorded back to back on the same sample and the same spectrometer there is a small difference in chemical shifts).
GHSQC, GHMQC Typically those are acquired decoupled, i.e. during data acquisition the entire C13 range has to be decoupled which requires a significant amount of energy, again resulting in sample heating, temperature gradients and overall some degradation of the quality of the spectrum.

So even if the digital resolution is extremely good all those factors listed above affect the reproducibility of the results and therefore it is incorrect to list data with a precision based on the digital resolution. Furthermore, computer programs, VNMRJ is no different, have a tendency to provide many digits in their output irrespective of significance. How many digits can be reproduced is not an exact science nor easy to figure out. The table below provides guidelines based on many years of experience and are to be seen as best case scenarios:


Parameter Nuclei Precision Examples Comments

Chemical Shifts from 1D spectra

H1 +/- 0.01 ppm, provided temperature of the experiment is controlled and mentioned 3.26 ppm
C13, P31 +/- 0.1 ppm, provided temperature of the experiment is controlled and mentioned 105.2 ppm
APT, 1D-C13, 1D-P31
Chemical Shifts from 2D spectra H1/H1 F2: +/- 0.02 ppm   F1: +/- 0.05 ppm 3.26 ppm
H1/C13 F2 (H1):  +/- 0.02 ppm 
F1(C13): +/- 0.2 ppm
3.26/105.2 ppm
Coupling Constants from 1D spectra H1
+/-  0.1 Hz 8.5 Hz  
Coupling Constants from 2D spectra H1/H1
+/-  0.5 Hz 8 Hz (4)

(1) There is no doubt that on a modern spectrometer signals can often be distinguished to a few ppb (0.001 ppm). Example: chemical shift A at 3.258 and B at 3.264 ppm. Rounding would result in two times 3.26 ppm which is not incorrect but also hides a shift difference that was clearly observed. A possible-work around is to provide a footnote to the NMR data that says something like "chemical shifts are reported to 3 decimal places where distinctions could be made but they are reproducible only to 2 decimal places." This is honesty without leaving out potentially valuable information.

(2) Same as in (1) just on a level like 105.18 vs. 105.23 ppm with the appropriate comment analogous to (1).

(3) In homonuclear experiments the chemical shifts have to be extracted parallel to F2 (the higher digital resolution axis) and not F1 which has far inferior digitization. If C13 shifts have to be taken from an indirect detection experiment (e.g. GHSQC, GHMQC) the precision is lower as indicated in the Table due to the lower digital resolution in F1 (= C13 axis).

(4) Coupling constants can be distorted significantly in 2D experiments due to a variety of reasons such as passive couplings, hence the +/- 0.5 Hz precision may not be attained. Also a large 2D data set needs to be acquired (which our default parameter sets do). Coupling constants extracted from 2D experiments should be identified as such in a data Table.

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