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Chemical shift tolerances (comparing standards to questioned spectra)
Babcock_Hall:
Has anyone seen any work that discusses what tolerances in the values of chemical shift are to be expected in different NMR spectra? This is obviously important in identifying an unknown sample when a spectrum has already been obtained of compound under study. Clearly differences in solvent and temperature would have to be taken into account in some way.
So far I have only done some quick literature searching, which uncovered one 2005 reference to protein NMR (PMID: 15949752 DOI: 10.1016/j.jmr.2005.03.020), but my present question does not concern proteins.
EDT
IF there have not been any studies on this, does anyone have a feeling about what the tolerance should be?
TheUnassuming:
I had to do a similar search in my previous lab when a series of compounds were showing significant enough of changes in chemical shift from batch to batch to cause concern. I don't remember finding anything of note, so if you find anything else let me know. In that case it turned out that the NMR signal for many of the protons on a given scaffold was highly concentration dependent, so we had to standardize the concentration we were going to collect all spectra at from then on.
Corribus:
--- Quote from: Babcock_Hall on April 01, 2019, 05:13:14 PM ---Has anyone seen any work that discusses what tolerances in the values of chemical shift are to be expected in different NMR spectra? This is obviously important in identifying an unknown sample when a spectrum has already been obtained of compound under study. Clearly differences in solvent and temperature would have to be taken into account in some way.
So far I have only done some quick literature searching, which uncovered one 2005 reference to protein NMR (PMID: 15949752 DOI: 10.1016/j.jmr.2005.03.020), but my present question does not concern proteins.
EDT
IF there have not been any studies on this, does anyone have a feeling about what the tolerance should be?
--- End quote ---
Are you talking instrument to instrument, or sample to sample on the same instrument?
Don't want to put a value on it, but I imagine if the magnet isn't maintained well (uniform field), the variation could be quite high from sample to sample.
pgk:
In theory, the proton shift depends on the overall deformation of the magnetic field that is caused by the molecule structure and the solvent interactions, as well as interaction with impurities and which are temperature dependent. Thus, the proton shift is the same, if being pure and taken in the same but pure solvent and the same temeperature; regardless the functional frequency of the instrument because the frequency affects the peak resolution and not the proton shift. Consequently, product/solvent impurities may be responsible for slight variations of the shift from sample to sample.
However, modern instruments lock the zero shift by software driven electronic devices, which are sensible to errors due to electric current fluctuations or magnetic interactions from e.g. a moving metallic carriage that is passing nearby. Thus, very slight variations of proton shift from instrument to instrument must also be expected.
PS: It is obvious that slight variations of the ternary protein structure, may cause significant variations of the peak shift and appearance due to long distance, intramolecular interactions.
Babcock_Hall:
--- Quote from: Corribus on April 02, 2019, 10:12:55 AM ---
Are you talking instrument to instrument, or sample to sample on the same instrument?
Don't want to put a value on it, but I imagine if the magnet isn't maintained well (uniform field), the variation could be quite high from sample to sample.
--- End quote ---
I am actually asking for a former student of mine. My understanding is that it was one instrument to another.
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