I am doing NMR studies of DNA - drug interacted complexes theoretically. While trying to compare it with the experimental results I am facing some difficulties. I want to know when an atom can be considered as shielding or deshielding in H, N and C nmr. Is there any range for the above nmr methods or is it depends on the functional groups. For eg: if the value of H - NMR is greater than 6 ppm then it can be considered as desheilding. If I am right what about C-NMR and N-NMR. Can you please clarify my doubts?
Sincerely,
R. Radhika.
Radical,
If you are looking for a kind of standard to state whether or not a proton for instance in the case of H-nmr is shielded or not, I don't think such standard exist; These are my reasons,
1. The chemical shift of a nucleus, whether H, C or N or P-nmr depend on many factors other than the actual shielding of such nucleus. In the case of H-nmr for example, the chemical environment, hydrogen bonding, solvent, type of substitutents, if there is an aromatic ring, in which case you consider mesmeric effects which supplies electron more at the ortho and para position and negative inductuctive effect which withdraws electron mostly at same position as well as present and absent of pie electrons. All these factors affects d chemical shift of a particular proton.
2. If u mark off say 6delta as d mid point, so that from 6 above will be protons that are deshielded and below are protons that are shielded, you will find out that such assumption will not work if u change your solvent. For instance, the proton of the OH of neat ethanol appears as a slightly broaden peak at delta 5.28 ppm but at 5-20% concentration in non-polar solvent, say CCl4, the proton resonates upfield between 2.0 and 4.0 and finally on infinite dilution when there is practically no hydrogen bonding, the same proton resonates near 0.5. So, if u had based ur mark of upfield and downfield using the first illustration, what would you say when you change your solvent?
3. What I know of, is the fact that you can say a nucleus is resonating upfield or downfield in relative to another nucleus in same compound. However, generally speaking, one can use the term like far upfield or downfield or slightly upfield or down field, depending on the position of the chemical shift.
Finally, a proton whose chemical shift is found far down field in a polar solvent might be found upfield in a non polar solvent and so, it will not make sense to say that this particular proton in this compound is highly shielded becos it resonated upfield in solvent A. This is because there are some other factors that affects the shift. Hope this helps but if I really didn't give you what you wanted, you can rephrase your question so I could get it proper. I just answered this based on what I think is what you are asking to know.
Regards
David
Shielding and deshielding are just terms relative to a given reference value. Indeed if one looks at some NMR data e.g. in the 1950s, they have a scale with zero on right and another with zero to the left (called a Tau scale). The idea with a Tau scale is that low electron shielding should be a lower number versus tetramethylsilane (TMS) which has close to zero shielding. The Tau scale was abandoned because every so often, someone would discover a peak below zero, and someone would rightly ask: what does a value below zero mean. Actually nothing because the zero in Tau has no discrete physical meaning. In contrast zero ppm in NMR terminology refers to an as non-polarized and as symmetrical as possible a chemical structure. So anyone who does an HNMR experiment, everyone will get an identical NMR spectrum when they report the result versus TMS. Every NMR frequency for any molecular species has one reference compound that has the highest (or nearly the highest) signal on the right. This has a very practical value as well as theoretical importance. If one of two singlet peaks in an NMR spectrum which has not previously been calibrated correctly, it can be highly confusing: one does not actually know which peak is the more shielded and which is the more deshielded.
I concur with David also that shielding and deshielding are affected by things other than individual atom types, e.g. solvent effects.
Best regards,
Walter.
Dear Dr. David,
Thank you so much for the valuable suggestion. Now I could understand shielding and deshielding is highly relative to many factors. so we could not generalize it. Actually I wanted to observe the difference in the chemical shift between the isomer and drug interacted complex of my system of interest (in gas phase). Here I have considered only the hydrogen bond formed in the complex. So I confused with the values. As per your suggestion, based on the result between the isomer and drug interacted complex I am going to conclude whether it is shielded or deshielded in that specific condition which I have considered and not in common (I may be wrong because not considered other effects). But here also the problem is, I have not done the experiment for the drug interacted complex. I know only the theoretical values of them (But I know the experimental values of the isomers which correlates well with my theoretical values ). So I want to know whether my approach is correct. Can I conclude about shielding/deshielding based on the theoretical results ?
Sincerely,
R. Radhika
Dear Dr.Walter Schmidt,
Thanks for the great explanation . Your suggestion "Shielding and deshielding are just terms relative to a given reference value" is so worthy for me in my work.
Sincerely,
R. Radhika
Dear Dr. Gordon,
I have used TMS as a reference molecule and have calculated the isotropic values correspond to the basis function. And thanks for the detailed clarification for my question. Here in an example, you have suggested like " If I have a signal at 2 ppm, a signal at 4 ppm is de-shielded (relatively speaking) and a signal at 0.5 ppm, is shielded by comparison (relative to the signal at 2 ppm)". Can you please clarify which values you have specified here (isotropic shielding values or chemical shift values)?
Dear Sir,
Chemical shift is the difference between the isotropic shielding values of reference molecule and the molecule of interest in theoretical calculation. So only I asked whether you have considered only the isotropic shielding of the molecule of interest or the chemical shift (difference in isotropic shielding ). Because I am comparing both the values. But I hope in your example you told about the chemical shift values. Am I right sir?
Sincerely,
R. Radhika
Dear Dr. Gordon,
Thank you for the suggestion. Surely, I will refer some books.
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