How to run quantitative 13C and 29Si NMR faster

Quantitative 13C NMR requires full relaxation of all carbons at each scan, which is typically very long (could mean that you have to set d1 to > 100 seconds).  Adding paramagnetic chemicals could speed up the relaxation.  A common choice is chromium acetylacetonate, or Cr(acac)3.  The trick is to add a suitable amount to your solution.  Too little, the 13C T1 will not be short enough.  Too much, the signal will be broadened too much (because T2 is shortened too much) and you will lose resolution.  Between 0.1wt% and 0.5wt% is typically used. The solution will have a light purple color.

The easiest way of avoiding the mess of weighing the Cr(acac)3 is to look at the color of the solution.  Following is a picture of a standard sample with 0.5wt% Cr(acac)3 for your reference.  Add Cr(acac)3 so that your sample has a similar or slightly lighter color than this.

0.5pct-cracac3

Then you should read in a different parameter set – C13CPD90 (or C13QUANT) – rather than the default one.  This will result in ca. twice as much signal as the default one.

The same applies to 29Si (and especially so since 29Si T1 is an even worse problem) but you should choose SI29IG, which is run without NOE since NOE of 29Si does not enhance signal.