Shielded and Deshielded Proton (1H-NMR)

  Shielded and Deshielded Proton (¹H-NMR)

Shielded and deshielded protons are used to determine the environment of molecules in NMR spectroscopy. Shielded simply means protective covering around atoms and deshielded without protective field/covering around a proton.

As far as we are aware, the fundamental idea behind NMR is to apply an external magnetic field termed B0 and detect the frequency at which a nucleus reaches resonance. A weak magnetic field produced by electrons in the nucleus opposes the B0. In this instance, we can argue that the electrons are protecting the nucleus from B0.

Shielded Protons

As we know NMR spectrum is formed on the basis of applied radio frequency against absorption. The position of signals in the spectrum tells the chemical shift value. This chemical shift or position of signals is affected by the nature of protons. Proton shielding and deshielding occur due to the electronegative atoms around the specific proton.

An opposing magnetic field to the applied field is produced by the proton's electrons. Because this minimizes the field experienced at the nucleus, the electrons are considered to protect the proton. As shown below,

Electronegative atoms attract electrons towards themselves more strongly than other simple atoms, we also know that electronegativity is the ability of an atom to attract the shared pair of electrons towards its own nuclei.

So, shielding is the ability of an atom to resist the external field around it.

In the case of the NMR spectrum, shielded protons are those protons that have electrons around themselves and these electrons work as a protective cover.

The effective nuclear charge of the nucleus exerts a strong attractive pull on each of its electrons. There is very little shielding between the nucleus and the electrons since all of the electron levels are drawn extremely close to it. The stronger the shielding, the higher the opposing magnetic fields to B0. from the electrons are relative to the nucleus' electron density. The chemical shift shifts up a field (lower ppm) because the proton receives a lower external magnetic field and hence requires a lower frequency to reach resonance.

 

Shielded Proton Behavior in NMR

Shielded protons have spin slight opposite to applied magnetic field, so they require low energy to bring them into resonance, upfield signal appear. when the applied magnetic field and proton magnetic field becomes equal in direction, signals appear (this is called resonance).

Deshielded protons

Deshielded protons don’t have electrons around them.

 

The density of electrons is low so, when a magnetic field is applied these protons feel more externally applied magnetic field. Their signals appear downfield in the spectrum.

The term "deshielded" refers to a nucleus that is more sensitive to the external magnetic field B0 as the electron density around it falls and the opposing magnetic field shrinks. The chemical shift shifts downfield (higher ppm) because the proton experiences a greater external magnetic field and hence requires a higher frequency to establish resonance.

Example:

The hydrogen nucleus will become unshielded as a result of the electronegative atom fluorine pulling the electron density towards it (electron withdrawing). This will cause an increase in the resonance frequency and a shift to higher ppm. In the case of CH4, the hydrogen nucleus is protected, hence the peak is on the lower ppm side.

 

Deshielded Proton Behavior in NMR 

A downfield signal or high value of sigma means these protons are deshielded and their spins are opposite to applied magnetic field, so they require high amount of energy for resonance.

Major differences between Shielded and Deshielded Protons

Shielded Proton	Deshielded Proton
Require less energy for flipping	Take more energy for flipping
Peak appear upfield	Peak formed downfield
Less β effective (applied magnetic field effect)	High β effective (applied magnetic field effect)
Signals nearer to TMS	Signals far from TMS
Low chemical shift value	High chemical shift value

 

The conclusion is that shielded protons absorb radiation at higher frequencies, whereas deshielded protons absorb at lower frequencies.

Position of signal in NMR