I was wondering whether anyone could help me to explain why the chemical shifts in my proton and carbon NMR results do not exactly match with those reported in the literature? The solvent used in my experiment is exactly the same as the one used in the literature of reference (CD3OD), however the frequency applied in my experiment was 500MHz as opposed to 400MHz by the study i am comparing my results with.
For example, in my proton NMR spectra, my results are usually 0.08 to 0.26 ppm higher than that in the literature (e.g: 5.16 x 5.00; 7.51 x 7.25). In the carbon NMR, the results, the variation goes from 0.01 to 3.58 ppm.
I wonder, is that acceptable? can I still mention that my results are in agreement with those reported in the literature?
I have looked everywhere and i couldnt find whether there are variation limits for comparison of NMR data.
Allright I'll try to answer. For small organic molecules 1H and 13C shifts must match precisely - on condition that sample temperature is the same in both measurements. How well is the temperature controlled in the two instruments? You might try to use a simple methanol NMR thermometer to test this.
Keep in mind that chemical shift is sensitive to temperature especially for protons participating in hydrogen bonds and when there is any chemical exchange.
Chemical shift itself can be perturbed if the molecule can be partially oriented by the magnetic field and chemical shift is highly anisotropic (depends on orientation of the molecule with respect to the magnet). This effect can be seen in nuclei with large spread of chemical shifts (31P, 13C) and mostly at very high fields.
Orientation of molecules can be perturbed by the magnetic field either directly - but this would take a special kind of molecule (it has to be highly asymmetric) - or by the presence of other things that are oriented by the field - like lamellar structures or phages.
There may be other exotic situations where actual chemical shift changes depending on the magnetic field alone, I hope that somebody else could fill in on this.
As Evgeny rightly says, the temperature of the sample is probably one of the main causes of chemical shift differences but there are others. Sample concentration can have an effect and pH can also have a major effect.
Observed chemical shift is due to dynamic processes which affect the environment of the nuclei. There is a combination of through-bond and through-space effects. What we observe is the average of these over the NMR timescale. As there is seldom a single solution conformation in most organic molecules, anything that affects the dynamics of these conformations will affect the chemical shift. Hence sensitivity to temperature, concentration and pH. Obviously pH will also have an effect electronically if there are acidic or basic sites in the molecule.
answered Apr 17 '10 at 02:01
The title of this q is confusing. You have not examined the same sample at different field strengths. In addition to the other factors mentioned here, a significant perturbation of shifts is widely observed when there is variation of water content, even in chloroform samples. The variation of shifts you are observing is not generally acceptable for high quality chemistry journals or for GMP, unless you also repeat the assignment procedure and report how you did it.
answered Aug 13 '12 at 08:36