7.9 : Atomic Nuclei: Types of Nuclear Relaxation

Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.

In spin–lattice or longitudinal relaxation, the excited spins exchange energy with the surrounding lattice as they return to the lower energy level. Among several mechanisms that contribute to spin–lattice relaxation, magnetic dipolar interactions are significant. Here, the excited nucleus transfers energy to a nearby magnetic dipole, usually a tumbling proton.

Spin–lattice relaxation occurs to restore the longitudinal magnetization to its equilibrium value and is characterized by the time constant, T₁, which indicates the average lifetime of a nucleus in the excited state. T₁ is also called the dipolar or dipole–dipole relaxation time and can range from 0.01 to 100 seconds for liquids. The value of T₁ depends on the factors such as the type of nucleus, the location of a nucleus within a molecule, the size of the molecule, and temperature.

Transverse relaxation, also called spin–spin relaxation, occurs when precessing nuclei fall out of phase, resulting in magnetization decay. Transverse relaxation is influenced by static dipolar fields and is usually faster than longitudinal relaxation. The relaxation times observed in typical NMR experiments range from 0.1 to 10 seconds. Additionally, the spin-lattice relaxation time, T₁, depends on the applied magnetic field, while T₂ is independent of it.

While the relaxation process is essential to prevent saturation and obtain a detectable signal, it also affects the intensity of the NMR signals. Generally, the intensity of the NMR signal is affected by T₁ relaxation, whereas shorter T₂ results in broadened NMR signals.