October 20, 2019, 06:08:04 PM
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Topic: Chemical shift tolerances (comparing standards to questioned spectra)  (Read 543 times)

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Offline Babcock_Hall

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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?
« Last Edit: April 01, 2019, 05:56:21 PM by Babcock_Hall »

Offline TheUnassuming

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Re: Chemical shift tolerances (comparing standards to questioned spectra)
« Reply #1 on: April 01, 2019, 05:50:11 PM »
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.
When in doubt, avoid the Stille coupling.

Offline Corribus

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Re: Chemical shift tolerances (comparing standards to questioned spectra)
« Reply #2 on: April 02, 2019, 10:12:55 AM »
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?

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.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline pgk

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Re: Chemical shift tolerances (comparing standards to questioned spectra)
« Reply #3 on: April 04, 2019, 12:56:33 PM »
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.
« Last Edit: April 04, 2019, 02:15:53 PM by pgk »

Offline Babcock_Hall

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Re: Chemical shift tolerances (comparing standards to questioned spectra)
« Reply #4 on: April 04, 2019, 05:36:47 PM »

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.
I am actually asking for a former student of mine.  My understanding is that it was one instrument to another.

Offline Babcock_Hall

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Re: Chemical shift tolerances (comparing standards to questioned spectra)
« Reply #5 on: April 05, 2019, 12:31:53 PM »
I asked an expert in NMR, and he pointed me to this site:  https://blog.acdlabs.com/my_weblog/2007/10/how-accurate-sh.html

Offline Babcock_Hall

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Re: Chemical shift tolerances (comparing standards to questioned spectra)
« Reply #6 on: April 13, 2019, 10:32:50 AM »
https://www.chem.wisc.edu/areas/reich/chem605/
Hans Reich wrote, "It is important to understand that the chemical shift of a given proton is not an invariant property of a molecule (like a melting point or boiling point), but will change depending on the molecular environment. The variability is especially large for NH and OH protons (several ppm), but even for CH protons reported shifts vary by a few tenths of a ppm. This is in part due to changes in measurement conditions, but additional variability in chemical shift is present in old NMR data (CW spectra) since spectrometer calibrations and spectrum referencing were not nearly as accurate as they are today. Nevertheless, if conditions are rigorously controlled, very high reproducibility of chemical shifts can be achieved. Databases of precise chemical shifts for many biomolecules have been created which facilitate simultaneous detection by NMR in aqueous solution."

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