Category Archives: knowledge bits

DEPT 135 Solvent Peak

On a DEPT-135 Spectrum, any carbons that are not bonded to protons will not have peaks.  Since solvent carbons are bonded to deuterium – not protons – those carbons will not have peaks either.

The protonated impurity in the solvent will have a small carbon peak on the DEPT135 spectra, but as a singlet rather than a multiplet.  And that peak will have a slightly different chemical shift than its deuterated counterpart due to isotope effect.

Isotope effect of chemical shifts

The 13C chemical shifts of CDCl3 and CHCl3 are different.  This is called the isotope effect, arising from the difference in mass for 1H and 2H.  So when you reference your spectrum using solvent 13C signals, be sure to use the chemical shift value of the deuterated version of the molecule.

This link gives a good overview of this effect.

When sample spinning helps and when not

Sample spinning helps to remove X and Y shimming imperfections.  This will improve resolution for routine 1D 1H and 13C spectra.  For any other advanced experiments, spinning is not recommended as it might introduce artifacts into your spectra.

When running routine 1H or 13C spectra, if the sample does not spin, you might want to touch up X and Y shims in addition to Z and Z2.  Please note that topshim for 400 only corrects Z – Z4 shims.  So, after topshim, if the lock still seems noisy, touch up X and Y.  Higher order transverse shims (XZ, YZ, XY etc) usually do not markedly affect resolution.

What if your samples do not have deuterated solvent

If your samples do not have deuterated solvent, you should run your spectra in unlocked state.  In bsmsdisp, uncheck lock (make the button white), then go to “lock” tab and uncheck “sweep on-off” button.

The spectra will not be of the highest resolution because you will not be able to shim using the 2H lock signal.  If resolution is a concern to you, you can shim by watching the FID.

Effect of Cl, Br, 14N, 31P, 19F etc to NMR spectra

Cl and Br have huge quadrupolar moments and their effect to other nuclei in the neighborhood can be considered non-existent.

The effect of 14N is dependent on its electron environment.  3-coordinated 14N has a large asymmetry in electron structure and thus the quadrupolar coupling is large, thus usually has negligible effect to neighboring nuclei.  4-coordinated 14N is more electronically symmetric and has a smaller quadrupolar coupling, therefore can split neighboring nuclei.  In this case, the neighboring 13C will be split into a triplet with intensity ratio of 1:1:1.  The splitting distance gives you the J-coupling constant between the nucleus of interest and the 14N.

2H has a fairly small quadrupolar coupling so it almost always split neighboring 13C to a 1:1:1 triplet.  Those who have run 13C spectra of samples with CDCl3 must be quite familiar with those triplets!

If you use acetone-d6 or dmso-d6 as solvent, the solvent peak on 13C spectra is not triplet but a septulet.  Do you know why and can you predict the intensity ratio of the 7 peaks?

31P and 19F have a spin of 1/2 and will always split neighboring nuclei to a 1:1 doublet.  If multiple 19F are present in the neighborhood, your 13C peaks will have a more complex splitting.