Thursday, February 28, 2008
Measuring 2H NMR Spectra
Occasionally students will come to the NMR lab with the need to measure the 2H NMR spectrum of a synthetic product they are attempting to deuterate. Every so often, one will come with their product dissolved in a deuterium labelled solvent. This is not a good idea for the same reason it is not a good idea to run the 1H NMR spectrum of a product dissolved in a protonated solvent. The solvent resonance will be orders of magnitude larger than the solute signal. When measuring the 2H NMR spectrum of a synthetic product, dissolve the product in a regular protonated solvent. Since there is no deuterated solvent, DO NOT ATTEMPT TO LOCK. You will have to shim the magnet as described here or use proton gradient shimming if possible. It is usually possible to observe the natural abundance 2H resonance of the solvent. This signal can be conveniently used as the chemical shift reference. The chemical shift scales are the same for 2H and 1H, therefore the natural abundance 2H chemical shift of the solvent is (to a very good approximation) equal to the 1H chemical shift of the same solvent. The figure below shows the 2H NMR spectra of two partially deuterated synthetic products run in chloroform and acetone. The natural abundance 2H signals of the solvents are highlighted in yellow.
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7 comments:
Not proud to admit it, but when I got an instruction on how to measure 2H NMR a while back in a different millennium, our NMR instructor asked me to prepare a demo sample for the occasion. Of course I dissolved the deuterated sample in CDCl3... and of course the instructor had not warned me beforehand, but let me walk right into the trap.
If I remember correctly we had to manually swap a few cable connections to use the lock-channel to record the 2H signal, and use a screwdriver to tune the channel, and so on.
Anyway, back to present: I do an 1H-decoupled 2H NMR and want to use a small amount (10 μL) of C6D6 as internal standard for quantification. Do you know anything about doing quantitative 2H NMR with deuterated internal standards? Can I expect good results?
Anonymous,
Provided that you respect the T1's, you should be able to get quantitative data.
Glenn
Dear Glenn Facey,
In my one of the NMR, the deuterated solvent was given wrong by the operator. For example, it was given D2O in place of CDCl3. Was that fine ? or I should ask them to record again with appropriate deuterated solvent. How does it affect the locking and chemical shift value ? Please explain !!
with thanks
VNM
VNM
See this post
http://u-of-o-nmr-facility.blogspot.ca/2008/05/consequence-of-locking-on-wrong-solvent.html
Glenn
Dear Glenn,
Thanks a lot for this form! I was just looking for some examples of T1 times of 2H nuclei. You mention T1's a s well. Do you have some numbers or at a range. What are save interscan delays?
With best regards, MS
MS,
The the most significant mechanism for 2H relaxation is quadrupolar rather than dipolar. The 2H T1 could be either longer or shorter than the T1 of a comparable proton. Unfortunately I do not have a table of 2H T1's on hand. It may be easier to just estimate it as described here https://u-of-o-nmr-facility.blogspot.com/2007/10/t1-measurements-and-estimation.html .
Glenn
Anonymous,
Here are some data on T1's and T2's (Table S1 in link below).
It's probably limited for what you need, as this is focused on in vivo 2H NMR. The values were measured on a preclinical 11.7 T MRI scanner.
http://advances.sciencemag.org/content/advances/suppl/2018/08/20/4.8.eaat7314.DC1/aat7314_SM.pdf
Henk
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