Concentration

¹³C signal strength is only 1/6000 of ¹H. For ¹H experiments, any reasonable concentration will give you sufficient signal, but for ¹³C, usually the higher the concentration, the better. Our 400MHz and 500MHz instruments are equipped with Prodigy® cryoprobes which boost ¹³C signal-to-noise ratio significantly.

Signal strength is proportional to the molar concentration of your sample. Generally, if you have ~ 10 mM, you should be able to obtain a decent ¹³C spectrum within half an hour.

How to run ¹³C faster and obtain quantitative result

See these entries:

How to run quantitative 13C and 29Si NMR faster

Power of Cr(acac)3 – a T1 relaxation agent

Some Useful Commands

• go:  Similar to zg, except that the new data will not overwrite the existing data, but add on to the existing data.  This is helpful when you have finished a run but feel your existing data does not have sufficient signal-to-noise ratio and you wish to add more scans.
• lb: line broadening parameter.  its unit is Hz.  Change lb, then do efp, and you will vary the resolution and signal-to-noise ratio of the spectrum.  The default lb is 0.3-1 Hz.  Larger lb suppresses noise but broadens the peaks.  Properly tuning lb value is very helpful when your sample peaks are naturally broad.
• tr: transfer the data acquired so far to your file, so that you can examine the preliminary spectrum when the acquisition is still running in the background.  This is helpful when you setup a long experiment.  Do tr after 20-30 scans, and look at the data.  If you see nothing (not even the solvent peak), something is wrong.  You want to find the problem early on so that you don’t waste a whole night’s time.

Variations of ¹³C experiments

DEPT gives you stronger signal than regular ¹³C, but will not show signals from non-protonated carbons.

Although HMQC is a 2D experiment, it is at least as quick as regular ¹³C (likely quicker). It also tells you which carbon is bonded to which proton.

HSQC is very similar to HMQC but has higher resolution and CH₂ show up as negative peaks (like that in DEPT135), which further helps your signal assignment.

Many advanced NMR (both 1D and 2D) techniques are as easy to run as a regular 1H or 13C on our spectrometers.  Interestingly, some of them are even faster than the regular 13C.  This entry describes an easy-to-follow instruction on how to set up those experiments.

The following document is a more complete description of how to set up and process 2D NMR experiments.  Beginners should skip the discussions on how to change various experimental parameters.  The default parameter settings are very easy to setup and should already take you a long way.  Once you feel comfortable running 2D using the default parameters, you will find it easy to adjust many parameters.

2D NMR handout

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, and you will experience hardship locking and topshim.  Between 0.4wt% and 0.6wt% (3 – 4 mg per 0.5 ml of solvent) is typically used. The solution will have a purple color.

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 color to this.

If your aim is to obtain a quantitative spectrum so you can integrate all peaks with confidence, you should read in parameter set CARBON1. If your aim is to obtain maximum signal strength, you should read in a different parameter set – C13CPD90 (or C13QUANT), which will result in ca. twice as much signal strength as the default CARBON1.

You will notice that the lock line gets quite noisy – this is normal; when chromium speeds up the T1 relaxation, it also speeds up the T2 relaxation, resulting in broader signals. The same effect is also applied to the solvent, resulting in noisier lock signal. If you have trouble doing Topshim, it is likely due to the noisy lock signal. Try manual shimming. 

The same applies to 29Si (and especially so since 29Si T1 is even longer than 13C T1) but you should choose SI29IG, which is run without NOE since NOE of 29Si does not enhance signal. You could also choose SI29QUANT. Note: for best results, you will need to add more Cr(acac)3 than 13C experiments. I recommend 8 mg per tube of 0.5 ml solvent.

This document discusses how to conduct T1, T2, and kinetics experiments.  It also covers the processing of data, which can be done in the Dynamics Center.

kinetics