Department of Chemistry,
University of Alberta March
NMR News 2007-01
News and tips from the NMR support group for users of the Varian NMR systems in the Department
Editor: Albin.Otter@ualberta.ca http://nmr.chem.ualberta.ca
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
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.
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:
(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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