Paramagnetic relaxation in 13C nmr

Question

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?

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