Thursday, May 15, 2008
The Consequence of Locking on the Wrong Solvent
Most often, running an NMR spectrum in automation requires only two pieces of information from the user, namely, the NMR experiment to be performed and the deuterated solvent used to dissolve the sample. With such a high demand on the user (this is meant to be scathingly sarcastic), there are bound to be errors in the input. If the user declares the wrong solvent, one of two things may happen. Firstly, the spectrometer may not be able to establish a deuterium lock and will report an error and not run the sample. Secondly, the spectrometer may be able to establish a lock despite the fact that the deuterium signal is off resonance. If the lock is established, the field strength will be set to a value appropriate to put the declared solvent signal on-resonance. When a proton NMR spectrum is collected, the chemical shift scale will be incorrect by an amount equal to the proton chemical shift difference between the true solvent and the declared solvent. This is illustrated in the figure below.In this case the solvent was benzene-d6. The lower trace shows the spectrum where the solvent was correctly declared as benzene-d6. The middle trace shows the spectrum collected when the declared solvent was CD2Cl2. Note that the spectrum is shifted to lower frequency by (7.16 ppm - 5.32 ppm = 1.84 ppm). The top trace shows the spectrum collected when the declared solvent was DMSO-d6. Note that the spectrum is shifted to lower frequency by (7.16 ppm - 2.50 ppm = 4.66 ppm). In this spectrum some of the resonances are outside of the spectral width and are lost, as the data were acquired with digital filtering. A similar thing can happen when a solvent with more than one deuterium signal (eg. THF-d8, methanol-d4, toluene-d8 ...) is correctly declared. The spectrometer may lock on the wrong deuterium signal of the solvent. In this case the chemical shift scale of a proton NMR spectrum will be incorrect by an amount equal to the chemical shift difference between the two types of protons in the solvent.