Thursday, February 28, 2008
Measuring 2H NMR Spectra
Occasionally students will come to the NMR lab with the need to measure the 2H NMR spectrum of a synthetic product they are attempting to deuterate. Every so often, one will come with their product dissolved in a deuterium labelled solvent. This is not a good idea for the same reason it is not a good idea to run the 1H NMR spectrum of a product dissolved in a protonated solvent. The solvent resonance will be orders of magnitude larger than the solute signal. When measuring the 2H NMR spectrum of a synthetic product, dissolve the product in a regular protonated solvent. Since there is no deuterated solvent, DO NOT ATTEMPT TO LOCK. You will have to shim the magnet as described here or use proton gradient shimming if possible. It is usually possible to observe the natural abundance 2H resonance of the solvent. This signal can be conveniently used as the chemical shift reference. The chemical shift scales are the same for 2H and 1H, therefore the natural abundance 2H chemical shift of the solvent is (to a very good approximation) equal to the 1H chemical shift of the same solvent. The figure below shows the 2H NMR spectra of two partially deuterated synthetic products run in chloroform and acetone. The natural abundance 2H signals of the solvents are highlighted in yellow.