Explain energy absorption in NMR

In an applied magnetic field, the two ¹H nuclear-spin states have slightly different energies. Absorption occurs when radiofrequency radiation matches this energy separation.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Explain energy absorption in NMR

Match radiofrequency to ΔE

In an applied magnetic field, the two ¹H nuclear-spin states have slightly different energies. Absorption occurs when radiofrequency radiation matches this energy separation.

You need to explain the relationship qualitatively; calculations of the transition energy are not required.

Keep three ideas separate: the applied field creates the gap, shielding adjusts the field experienced by a particular proton, and the radiofrequency supplies the matching photon energy.

Resonance

Resonance is absorption when radiofrequency photon energy matches the separation between field-split nuclear-spin states.

The transition takes α to β; relaxation subsequently returns population toward equilibrium.

Absorption promotes a nucleus from the lower α state to the higher β state. Resonance describes the matching condition; it does not mean the nucleus absorbs every radiofrequency in a broad range.

Field, shielding and frequency

Increasing effective field increases ΔE and the matching frequency. Electrons shield a nucleus so chemically different protons experience slightly different effective fields.

A signal requires a population difference; equal upward and downward transition rates with equal populations would give no net absorption.

If the transmitter frequency is swept, absorption occurs at the value that matches the effective-field gap. If the applied field is changed, the matching frequency changes with it.

After excitation, relaxation restores the small equilibrium population difference and transfers energy to the surroundings. Relaxation is distinct from the initial absorption step.

Explain a matching-frequency change

Compare the same proton at two applied magnetic fields. The stronger field produces a larger separation between the lower α and higher β spin states.

Because the absorbed radiofrequency photon must match that separation, the stronger-field spectrum requires a higher resonance frequency.

On the two diagrams, label the lower-field gap ΔE₁ and the stronger-field gap ΔE₂. The visual comparison ΔE₂ > ΔE₁ is enough to justify f₂ > f₁ through ΔE = hf.

Try this

Use an energy-level diagram to explain why the same proton resonates at a higher frequency in a stronger magnetic field.

Check your answer

Show a larger α–β separation at stronger field and link that larger gap to a higher matching radiofrequency.

Predict a field change

Supplied change: B₀ is increased for the same ¹H nucleus.

First link B₀ to the energy gap ΔE between the two spin states, then use ΔE = hf.

Use proportional language rather than inventing a numerical gap.

Try this

State the frequency response when B₀ increases.

Check your answer

A larger B₀ increases the splitting ΔE between the α and β spin states. Because ΔE = hf, the frequency that matches the gap increases.

Use shielding qualitatively

Supplied comparison: two ¹H environments in the same molecule at fixed B₀, one electron-rich (more shielded) and one next to an electronegative atom (deshielded).

Decide which proton experiences the larger effective field, then which needs the higher resonance frequency.

This prepares the reasoning for chemical shift without requiring transition-energy calculations.

Try this

Compare effective field and resonance for two differently shielded protons.

Check your answer

The more shielded proton experiences a lower effective field, because its surrounding electrons oppose B₀ more, so its spin-state gap and resonance frequency are slightly lower. The deshielded proton resonates at a slightly higher frequency. Both are ¹H; no different isotope is involved.

Intensity cannot replace energy matching

Increasing radiofrequency power supplies more photons but does not make a photon of the wrong frequency match ΔE.

Resonance remains frequency-selective.

Power and frequency play different roles. More power can provide more photons, but only photons with the matching energy can drive the stated transition.

Try this

Better reasoning: “Any radiofrequency is absorbed if the pulse is intense enough.”

Check your answer

The photon energy must match the spin-state gap; increasing intensity cannot compensate for the wrong frequency.

From resonance to chemical shift

After the check questions, explain a field-scaling example without notes. Return later for a different shielding question.

Try this next: interpret chemical shift.

A full answer should name α and β, show an upward absorption arrow, write ΔE = hf and explain the direction of any field or frequency change.

Try this

Annotate a resonance energy diagram with B₀, ΔE, hf and α→β.

Check your answer

A strong answer should include exact matching, upward absorption and the nuclear-state boundary.