UMass Nuclear Magnetic Resonance (NMR) Labs

This workshop is suitable for students with at least several months of experimental NMR experience.

workshop handout

NMR spectroscopy is a well-established scientific discipline.  Many scholars in the field have developed teaching materials that help new users get familiarized with the basics of NMR.  This NMR Wiki site collects links to many of these materials.  They all teach the same science but with their unique approach, emphasis, and target audience.  Pick an author that fits your taste, background, and needs.

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.

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.

This handout discusses several essential issues for 13C experiments.

This document describes 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.

Spectral Database for Organic Compounds SDBS offers the spectral data and interpretations of a wide selection of organic molecules.

This University of Wisconsin website by Hans Reich offers a nicely tabulated collection of chemical shifts and J-couplings of many molecules.

SDS has a webpage for chemical shifts of solvents.

The paper by Hugo E. Gottlieb, Vadim Kotlyar, and Abraham Nudelman in J. Org. Chem., 1997, 62 (21), 7512-7515 has an extensive list of chemical shifts of common solvents as trace impurities in other solvents.

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.

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.

Quantitative 13C NMR requires full relaxation of all carbons at each scan, which is typically very long (could mean d1 of > 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 default 13C standard data file uses NOE to enhance signal intensity but this would make the spectra less quantitative.  To create a quantitative 13C file, you will need to type rpar and select C13IG.  Then you must type getprosol to load the pulsing parameters.