NMR relaxation — naming the process from environmental magnetic field interactions What is the name for the relaxation process driven by interactions between an excited nucleus and fluctuating magnetic fields from surrounding molecules (the “lattice”) moving in the sample?

Introduction / Context:
NMR signals decay because excited nuclear magnetization returns to equilibrium through relaxation. Two principal mechanisms are T1 (spin–lattice) and T2 (spin–spin). Distinguishing them is pivotal for designing pulse sequences, interpreting contrasts in MRI, and understanding molecular dynamics that modulate local magnetic fields.

Given Data / Assumptions:

• T1 processes exchange energy between spins and their surroundings (“lattice”).
• Fluctuating local fields at or near the Larmor frequency enable transitions.
• T2 processes dephase transverse magnetization via spin–spin interactions without necessarily exchanging energy with the lattice.

Concept / Approach:

The description specifically points to environmental magnetic fields produced by molecular motion that drive recovery of the longitudinal magnetization (Mz) back to equilibrium. This is the definition of spin–lattice relaxation (T1). Spin–spin relaxation (T2) concerns mutual interactions among spins that cause phase dispersion of Mxy and loss of coherence, not primarily energy exchange with the surroundings.

Step-by-Step Solution:

Verification / Alternative check:

Inversion-recovery experiments measure T1 by tracking Mz recovery; results depend on molecular tumbling rates, confirming lattice-driven relaxation.

Why Other Options Are Wrong:

Spin–spin (B) is dephasing of transverse magnetization (T2), not energy exchange. Option C is not a standard term. “None” (D) is incorrect because T1 is well defined.

Common Pitfalls:

Assuming T2 processes restore longitudinal magnetization or conflating chemical exchange with T1 without considering spectral density at the Larmor frequency.

Final Answer:

Spin - lattice relaxation