Atomic Nuclei: Types of Nuclear Relaxation

Relaxation in NMR systems is a first-order exponential decay and can occur by either spin–lattice or spin–spin mechanisms.

Spin–lattice or longitudinal relaxation occurs primarily via magnetic dipole–dipole interactions with the surroundings, where the excited nucleus transfers energy to a nearby magnetic dipole, usually a tumbling proton.

Spin–lattice relaxation restores the Boltzmann distribution, and the spin–lattice relaxation time, T₁, indicates the average half-life of a nucleus in the excited state.

For liquids, T₁ values can range from 0.01 to 100 seconds, depending on the type of nucleus, its location within a molecule, the size of the molecule, and temperature.

Spin–spin or transverse relaxation occurs in the transverse plane when the spin-spin interaction between the precessing nuclei causes dephasing.

Spin–spin relaxation is governed by the time constant T₂, which is usually shorter than T₁.

While relaxation is essential to prevent saturation and obtain a detectable signal, high relaxation rates result in line broadening. The ideal half-life for an excited nucleus ranges from 0.1–10 seconds.