Shielding and Deshielding

Introduction

If NMR simply detected hydrogen nuclei, every proton would appear at the same frequency and the technique would be useless for structure determination. The reason signals spread across a spectrum is that electrons around each nucleus alter the magnetic field it experiences. Some protons are wrapped in a thick blanket of electron density and feel a weaker field; others have electron density pulled away and feel more of the applied field. These effects, called shielding and deshielding, are the physical basis of chemical shift and of every chemical shift table.

Core explanation

Electrons generate an opposing field. When a molecule is placed in the applied field B₀, the electrons around each nucleus are induced to circulate. Moving charge creates its own magnetic field, and for the electrons around a nucleus this induced field points against B₀. The nucleus therefore experiences a slightly smaller effective field:

B eff = B₀(1 − σ)

where σ, the shielding constant, is very small, of the order of millionths. This is why chemical shifts are measured in parts per million.

Shielded protons. A proton surrounded by high electron density has a large σ. It feels a smaller effective field, so the gap between its spin states is smaller and it resonates at a lower frequency. On the spectrum, it appears at low δ , to the right, upfield . Protons in alkanes, such as those in CH₃ groups, are fairly well shielded and appear around δ 0.9.

Deshielded protons. If electron density around a proton is reduced, σ decreases and the proton is deshielded . It feels more of the applied field, has a larger energy gap and resonates at a higher frequency, appearing at high δ , to the left, downfield .

What removes electron density? The most important cause is an electronegative atom nearby. Oxygen, nitrogen and the halogens pull electron density through the σ bonds towards themselves, away from carbon and its hydrogens. The effect is strongest on protons on the carbon directly attached to the electronegative atom and falls off rapidly with distance. Carbonyl groups and aromatic rings also deshield nearby protons, partly by electron withdrawal and partly by special magnetic effects of π electrons.

Summary of the chain of cause and effect.

more electron density → more shielding → smaller B eff → lower frequency → lower δ (upfield) less electron density → deshielding → larger B eff → higher frequency → higher δ (downfield)

Signals at the same δ. Protons with identical environments experience identical shielding and resonate together; this is why equivalent protons give one signal. Differences of just a few hundredths of a ppm are enough to separate protons in slightly different surroundings.

Carbon-13 follows the same logic. Carbon nuclei are shielded by their electrons too. A carbon bonded to oxygen, or a carbonyl carbon, is strongly deshielded and appears far downfield on the ¹³C scale.

Formulae

B eff = B₀(1 − σ). Resonance frequency is proportional to B eff, so a larger σ gives a lower frequency and a smaller δ.

Step-by-step reasoning

To predict which of two protons has the higher δ:

1. Identify the atoms and groups close to each proton. 2. Decide which proton has electron density withdrawn more strongly, for example by being nearer to O or a halogen. 3. That proton is less shielded, feels a stronger effective field and resonates at a higher frequency. 4. It therefore appears at higher δ, further downfield.

Visual explanation

Draw a proton nucleus in a cloud of electrons with a large arrow for B₀ pointing up and a small arrow for the induced field pointing down. Next to it, draw a proton with a thinner cloud because a neighbouring oxygen has pulled density away; its opposing arrow is shorter, so more of B₀ gets through.

Real-world analogy

Sunglasses reduce the light reaching your eyes. A proton wearing thick "electron sunglasses" feels a weaker magnetic field; remove some of the tint, as an electronegative neighbour does, and more of the field gets through.

Real-world example

In the proton spectrum of methanol, the CH₃ protons appear at about δ 3.4, whereas the CH₃ protons of ethane appear at about δ 0.9. The attached oxygen withdraws electron density, deshielding the methyl protons and moving their signal about 2.5 ppm downfield.

Why?

Why does the induced field oppose the applied field? Electrons circulating in response to a changing or applied magnetic field set up a field in the direction that opposes the one causing it, as described by Lenz's law. This diamagnetic response is present in every molecule and is the origin of shielding.

Common misconception

"Deshielded protons appear to the right because they are less protected." The δ axis increases to the left, so deshielded protons appear to the left at higher δ. It helps to remember that TMS, the most shielded common reference, sits at the far right.

Worked example

Question: Predict the order of chemical shift, lowest first, for the CH₃ protons in CH₃CH₃, CH₃Cl and CH₃F.

Reasoning: Electronegativity increases in the order C < Cl < F, so electron withdrawal from the methyl group increases in the same order. More withdrawal means more deshielding and a higher δ.

Answer: CH₃CH₃ (about 0.9) < CH₃Cl (about 3.1) < CH₃F (about 4.3).

Quick check

1. A proton's surrounding electron density is reduced. What happens to its resonance frequency and its δ value? Answer: It is deshielded, so its resonance frequency increases and its δ value becomes larger, moving downfield.

Exam focus

Use precise language: electrons shield the nucleus from the applied field; electronegative atoms withdraw electron density, deshielding nearby protons and moving signals downfield to higher δ. Keep the direction of the axis in mind and connect shielding to the position of TMS at δ 0.

Advanced insight

Shielding has two components: a diamagnetic term from electron circulation, which dominates for hydrogen, and a paramagnetic term arising from low-lying excited electronic states. The paramagnetic term dominates for heavier nuclei such as ¹³C, which is one reason ¹³C shifts span about 220 ppm while ¹H shifts span only about 12.

Summary

Electrons around a nucleus circulate in the applied field and produce an opposing field, so the nucleus experiences B eff = B₀(1 − σ). High electron density means more shielding, a lower resonance frequency and a low δ (upfield). Withdrawal of electron density, especially by electronegative atoms, deshields protons, raising frequency and δ (downfield). This link between electron density and signal position explains chemical shift tables.

Practice questions

1. Explain what is meant by the term shielding in NMR. Answer: Electrons around a nucleus create an induced magnetic field opposing the applied field, so the nucleus experiences a smaller effective field. 2. Why do the CH₂ protons in CH₃CH₂OH appear at higher δ than the CH₃ protons? Answer: The CH₂ group is bonded directly to oxygen, which withdraws electron density and deshields those protons more than the more distant CH₃ protons. 3. State whether a highly shielded proton appears upfield or downfield, and explain. Answer: Upfield, at low δ, because it feels a smaller effective field and resonates at a lower frequency. 4. Put the CH₃ protons of CH₃Br, CH₃I and CH₃Cl in order of increasing δ. Answer: CH₃I < CH₃Br < CH₃Cl, following the increasing electronegativity of the halogen.