Noyaux atomiques : fréquence de précession de Larmor

Larmor precession is the gyration of a spinning nucleus's magnetic moment about an applied magnetic field, describing a cone-shaped path. Similar to how Earth's axis wobbles due to gravitational forces, a nucleus's magnetic moment precesses when exposed to an external magnetic field directed along the z-axis. This fundamental phenomenon is central to nuclear magnetic resonance.

The Larmor frequency, or precessional frequency (ω), is directly proportional to the applied magnetic field (B0). The magnetogyric ratio (γ) serves as the proportionality constant, connecting these two quantities. For hydrogen nuclei, γ equals 2.675 × 10⁸ T⁻¹ s⁻¹, making this relationship essential for NMR instrument operation and frequency calculations.

The magnetogyric ratio (γ) is the proportionality constant between precessional frequency and magnetic field strength, fixed for each nucleus. Each nucleus has a unique γ value because it reflects the specific ratio of magnetic moment to angular momentum, determined by the nucleus's charge and mass composition. This uniqueness enables selective excitation in NMR spectroscopy.

The Larmor frequency (ω) can be expressed in terms of the operating frequency (ν) of the NMR instrument. By rearranging the relationship between ω and B0 and multiplying by Planck's constant, the equation yields the connection between Larmor frequency and the energy difference between spin states (ΔE), fundamental to understanding NMR signal generation.

A spinning nucleus precesses in a magnetic field because the magnetic moment experiences a torque perpendicular to both the moment and the applied field. This torque causes the magnetic moment vector to trace a cone around the magnetic field axis, analogous to how gravitational forces cause a spinning top to wobble. The precession continues as long as the external magnetic field is applied.

The Larmor frequency determines the energy difference (ΔE) between nuclear spin states. By expressing the precessional frequency in terms of NMR operating frequency and multiplying by Planck's constant, the energy gap between spin states becomes proportional to the Larmor frequency. This relationship explains why higher magnetic fields produce larger energy separations and stronger NMR signals.

Both Earth's axis and a nucleus's magnetic moment precess due to external forces perpendicular to their angular momentum. Earth's axis wobbles because of gravitational pull from the sun and moon, while a nucleus's magnetic moment precesses due to an applied magnetic field. Both phenomena involve the same geometric principle: a spinning object tracing a cone around a reference axis.