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Shielding in H-NMR

The structural factors mean that different types of proton will occur at different chemical shifts. This is what makes NMR so useful for structure determination, otherwise all protons would occur at the same frequency, limiting the information to the presence of H.

The various factors that influence the field include:

Electronegativity

The electrons around the proton create a magnetic field that opposes the applied field. Since this reduces the field
experienced at the nucleus, the electrons are said to shield the proton. It can be useful to think of this in terms of vectors....

magnetic field of electrons shields the nucleus from the full effect of the applied field

plot of electronegativity vs chemical shift for CH3-X Since the field experienced by the proton defines the energy difference between the two spin states, the frequency and hence the chemical shift, δ /ppm, will change depending on the electron density around the proton. Electronegative groups attached to the C-H system decrease the electron density around the protons, and there is less shielding (i.e. deshielding) so the chemical shift increases. This is reflected by the plot shown in the graph to the left which is based on the data shown below.

 
Compound, CH3X
CH3F
CH3OH
CH3Cl
CH3Br
CH3I
CH4
(CH3)4Si
X
F
O
Cl
Br
I
H
Si
Electronegativity of X
4.0
3.5
3.1
2.8
2.5
2.1
1.8
Chemical shift, δ / ppm
4.26
3.4
3.05
2.68
2.16
0.23
0

 
These effects are cumulative, so the presence of  more electronegative groups produce more deshielding and therefore, larger chemical shifts.
Compound CH4 CH3Cl CH2Cl2 CHCl3
δ / ppm 0.23 3.05 5.30 7.27
These inductive effects are not just felt by the immediately adjacent protons as the disruption of electron density has an influence further down the chain.  However, the effect does fade rapidly as you move away from the electronegative group.  As an example, look at the chemical shifts for part of a primary bromide
H signal
-CH2-CH2-CH2Br
δ / ppm  1.25  1.69  3.30

Magnetic Anisotropy

The word "anisotropic" means "non-uniform".  So magnetic anisotropy means that there is a "non-uniform magnetic field". Electrons in π systems (e.g. aromatics, alkenes, alkynes, carbonyls etc.) interact with the applied field which induces a magnetic field that causes the anisotropy.  As a result, the nearby protons will experience 3 fields: the applied field, the shielding field of the valence electrons and the field due to the π system. Depending on the position of the proton in this third field, it can be either shielded (smaller d) or deshielded (larger d), which implies that the energy required for, and the frequency of the absorption will change.

Hydrogen Bonding

Protons that are involved in hydrogen bonding (this usually means -OH or -NH) are typically observed over a large range of chemical shift values.  The more hydrogen bonding there is, the more the proton is deshielded and the higher its chemical shift will be. However, since the amount of hydrogen bonding is susceptible to factors such as solvation, acidity, concentration and temperature, it can often be difficult to predict.

HINT : It is often a good idea to leave assigning  -OH or -NH resonances until other assignments have been made.

Experimentally, -OH and -NH protons can be identified by carrying out a simple D2O (deuterium oxide, also known as heavy water) exchange experiment.  These H atoms are said to be exchangeable.

Why would a peak disappear ?

Consider the alcohol case for example:   R-OH  +  D2O   <=>   R-OD   +   HOD
During the hydrogen bonding, the alcohol and heavy water can "exchange" -H and -D each other, so the alcohol becomes R-OD.
Although D is NMR active, its signals are of different energy and are not seen in the H-NMR, hence the peak due to the -OH disappears.  (Note that the HOD will appear...)

QUESTION

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary