I'm trying to make my 13C{1H} (hydrogen decoupled) nmr more quantitative. From the literature I've noticed that researchers either

  1. increase the delay time in the experiment (which I can't do, as it takes too long to collect the spectra)
  2. run the decoupling pulse only during data acquisition (to reduce the nuclear Overhauser effect), and/or
  3. add a paramagnetic relaxation agent to the sample.

I want to try a paramagnetic relaxation agent (I'm working with $\ce{CDCl3}$ as a solvent). The most common one seems to be $\ce{Cr(acac)_3}$. Are there better relaxation agents out there (i.e. more effective at lower concentration)? Also, the literature I've read shows all sorts of concentrations of the relaxation agent... would a low (0.01 M or less) concentration work, in your experience? Do these agents lead to too much spread in the peaks?

Also, will using a relaxation agent ruin the nmr tube for further use?


1 Answer 1


Paramagnetic relaxation in $\mathrm {~^{13}C}$ NMR:

Quantitative $\mathrm {~^{13}C}$ NMR requires full relaxation of all carbons at each scan. Adding paramagnetic relaxation reagents could speed up the relaxation. The most common reagent you can find online is chromium acetylacetonate, $\ce{Cr(acac)3}$. However, the amount you need to add to your solution is very critical. It is tricky that if your added amount to your solution is too little, $\mathrm T_1$ of the $\mathrm {~^{13}C}$ will not be short enough. I you added too much of $\ce{Cr(acac)3}$, the signal will be too much broadened due to too much shortening of $\mathrm T_2$, thus you will lose resolution. Typically used recommended amount by experts is Between $\mathrm{0.1wt\%}$ and $\mathrm{0.5wt\%}$ (https://blogs.umass.edu/weiguoh/?p=71). With that amount, the solution will have a light purple color [1] (courtesy of https://blogs.umass.edu/weiguoh/?p=71).


Above picture is a picture of a standard sample with $\mathrm{0.5wt\%}$ $\ce{Cr(acac)3}$, which is an excellent reference for you. It'd provide you the easiest way to avoid the mess of weighing $\ce{Cr(acac)3}$. What you have to do is to look at the color of your solution. Following the color of the picture, you can add $\ce{Cr(acac)3}$ a little by little so that your sample has a similar or slightly lighter color than this.

To read about othe reagents download this thesis (available online): https://www.diva-portal.org/smash/get/diva2:793434/FULLTEXT01.pdf


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.