Table of Contents
Chemical shift is the phenomenon used in NMR spectroscopy. You need to first understand what is chemical shift to understand the factors affecting chemical shift.
There are four factors that affect the value of chemical shift (δ) in NMR spectroscopy.
- Electronegativity
- Anisotropic effect
- Van Der Waals deshielding
- Axial and equatorial effect
I will explain these factors in detail.
Electronegativity
This effect is a major factor that influences chemical shift.
As you know that a proton is surrounded by an electron cloud which shields the nucleus from an applied magnetic field.
When we have an electronegative group in the molecule, it withdraws the electron cloud towards itself and the proton gets deshielded and experiences more magnetic field. This is the electronegativity effect on the chemical shift.
On the contrary, when we have electropositive or electron-donating groups in the molecule, we get a shielding effect and the nucleus thus experiences less magnetic field.
More electronegativity – more deshielding – resonance at a higher frequency – more will be the difference between resonance frequencies of the sample and the internal standard – more will be the chemical shift (δ) value.
Anisotropic effect
The anisotropic effect is another major factor that influences the chemical shift.
When the structure has π electrons like in alkenes and alkynes, an anisotropic effect is seen.
The π electrons are loosely held and therefore they have their own momentum and they generate tiny magnetic moments. When such a generated magnetic moment is aligned with the applied magnetic field with respect to the proton, we observe a paramagnetic effect that is similar to deshielding. Then the chemical shift value becomes larger.
When the generated magnetic moment is opposed to the applied magnetic field with respect to the proton, we observe a diamagnetic effect that is similar to shielding. Then the chemical shift value becomes smaller.
We can explain the anisotropic effect with the help of examples such as alkynes, alkenes, benzene, aldehydes, and carboxylic acid. I have explained the anisotropic effect of alkyne and alkene below.
Van der Waals deshielding
Van der Waals deshielding is a minor factor affecting chemical shift. In fact, this causes the change in chemical shift value only up to 1δ. This effect is seen in some compounds that are very substituted like substituted steroids and alkaloids.
In rigid molecules, protons occupy a sterically hindered position. As a result, the electron cloud of the hindering group repels the electron cloud surrounding the proton by electrostatic repulsion. Hence, the proton gets deshielded and appears at a larger chemical shift value than anticipated.
Axial and equatorial effect
This effect is similar to the anisotropic deshielding but it is not as significant as that effect.
A classic example of this effect is seen in cyclohexane where the equatorial protons come to resonance about 0.5 ppm higher than the axial protons. This happens because the equatorial proton comes in the deshielding zone and the axial proton comes in the shielding zone.
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