Spin-1/2 absorption in NMR — what changes upon RF excitation? When a spin-1/2 nucleus in a magnetic field absorbs resonant radiation energy, which change best describes its magnetic moment relative to the applied field B0?

Introduction / Context:
In NMR, nuclei with spin-1/2 (e.g., 1H, 13C, 19F) occupy one of two Zeeman energy levels in a static magnetic field B0: a lower-energy state aligned with the field and a higher-energy state opposed to it. Absorption of radiofrequency (RF) energy at the Larmor frequency induces transitions between these states and tips the net magnetization away from equilibrium.

Given Data / Assumptions:

• B0 defines the quantization axis and causes Larmor precession.
• At thermal equilibrium, more spins populate the lower-energy (parallel) state.
• RF pulses at resonance transfer population to the higher-energy (anti-parallel) state and rotate the net magnetization.

Concept / Approach:

Absorption corresponds to promotion from the parallel to anti-parallel state for spin-1/2 nuclei. Classically, this appears as the magnetic moment vector tipping to oppose B0 (for a 180-degree inversion) or being rotated into the transverse plane (for a 90-degree pulse). The Larmor frequency itself is set by gamma and B0 and does not permanently increase; mechanical spin speed is not what changes—quantum state populations do.

Step-by-Step Solution:

Verification / Alternative check:

Inversion recovery and saturation experiments demonstrate controllable population changes (and signal phase), consistent with flips between Zeeman levels rather than changes in B0 or intrinsic mechanical spin.

Why Other Options Are Wrong:

Option A confuses excitation with altered Larmor frequency. Option B misinterprets quantum transitions as changes in mechanical spin rate. Option E is physically incorrect; B0 is constant. “None” (D) is unnecessary because C is correct.

Common Pitfalls:

Equating precession frequency shifts with excitation; overlooking the distinction between state populations and classical analogies.

Final Answer:

The angle of precession flips so that the nuclear magnetic moment opposes the applied field (transition to the higher-energy state)