Can someone explain me why different chemical groups (amino group, carbon group etc) have different values of chemical shift in NMR Spectrum? I think it depends on electronegativity but I have not clearly understood it yet.
different chemical shifts arise due to two reasons, first, nuclei associated with different isotopes have different gyro-magnetic ratios in general, hence different Larmor frequencies when subjected to the same external magnetic field, i.e, the Zeeman field, secondly, even nuclei of same isotope but in different chemical environment do not experience the same Zeeman field but higher or lower values than it due to shielding/de-sheilding effects of the neighborhood electron cloud surrounding the nucleus.
a very detailed discussion of the mechanisms of origin of chemical shifts is provided in the Spin Dynamics Book by Prof. Malcolm Levitt.
Hi Thanasis,
this question cannot be simply answered in a few sentences but is rather subject to text books. There are many very nice examples and they are worth to be read.
AXEL
Mr. Thanasis,
In general, the answer to your question is: Yes, due to, electronegatvity.
Agreeing with Axel Klein, this is a big subject and is treated in many textbooks on NMR. For example there are electronegativity effects when an electron withdrawing group reduces the diamagnetic shielding experienced by a particular nucleus. There are ring current effects for nuclei on aromatic groups. It is best to refer to a textbook dealing with chemical applications of NMR. It is of course the differences in chemical shifts for particular types of nuclei that make NMR useful for identifying chemical species.
Basically two important factors cause chemical shifts:
1. Electron Density: In general, higher electron density lower chemical shift lower electron density higher chemical shift
High electron density of neighboring atoms may shield the nucleus from an external magnetic field. If there is little or no electron density in near by atoms or groups, the nucleus can also be deshielded.
2. Electronegativity: The higher the electronegativity difference between H and its surrounding atoms, the higher the chemical shift. This is because the higher the electronegativity difference between H and another atom, the lower the electron density is around the H (proton), and thus the more deshielding the proton experiences. As a result, the proton will have a higher chemical shift.
The more details information are provided in below source.
Source
http://www.chem.ucla.edu/~harding/ec_tutorials/tutorial39.pdf
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