7.7: Atomic Nuclei: Magnetic Resonance

The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the lower to the higher energy state, This causes the net magnetization to shift from the z axis towards the y axis. On withdrawing the applied radiation, as the nuclear spins lose the absorbed energy and return to the spin-up state, the net magnetization vector returns to its orientation along the z axis, and equilibrium is established. All NMR-active nuclei exhibit nuclear magnetic resonance, which forms the basis of NMR spectroscopy and imaging.

The small majority of nuclear spins aligned in the lower energy state represents the excess population.

As the spins precess about the B₀ field at the Larmor frequency, ω, the sum of their magnetic moments results in a net magnetization about the z axis.

When a pulse or a short burst of radio waves is applied along the x axis, the nuclei absorb energy corresponding to their Larmor frequency.

The coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the lower to the higher energy state, shifting the net magnetization towards the y axis.

On withdrawing the radiation pulse, the nuclear spins lose the absorbed energy. The net magnetization vector shifts back to the z axis, and equilibrium is established.

All NMR-active nuclei exhibit nuclear magnetic resonance, which forms the basis of NMR spectroscopy and imaging.

• Nuclear magnetic resonance (Main keyword) - A technique that exploits the magnetic properties of certain atomic nuclei for spectroscopy and imaging.
• Atomic resonance - The phenomenon of an atom's nuclei oscillating at specific frequencies under energy influence.
• Nuclear magnetization - The net magnetization oriented along an axis due to the excess nuclear spin population.
• Larmor frequency - The frequency at which nuclear spins precess in an external magnetic field.
• NMR-active nuclei - Those atomic nuclei which exhibit nuclear magnetic resonance.

• Define Nuclear magnetic resonance - Explain what it is (e.g., nuclear magnetization of atomic nuclei).
• Contrast Nuclear Magnetization vs Atomic Magnetization – Explain key differences (e.g., orientation of nuclei in an external magnetic field).
• Explore Examples – Describe scenario (e.g., application of radio waves at the Larmor frequency causes atomic resonance and change in nuclear spin).
• Explain Process – Magnetic influence on atomic nuclei and the resulting resonance.
• Apply in Context – How nuclear magnetic resonance contributes to spectroscopy and imaging.

• What is nuclear magnetic resonance and how does it cause atomic resonance?
• How is nuclear magnetization oriented in an external magnetic field?
• What is the role of the Larmor frequency in atomic resonance?

• Students – Understand How nuclear magnetic resonance can provide insight into chemical structures.
• Educators – Provides a clear framework it helps with teaching atomic structure and interactions.
• Research Scientists – Relevance in the study of biomolecular chemistry and medical imaging.
• Physics Enthusiasts – Offer insights into the interaction of magnetic fields with atomic nuclei.