7.9:
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.
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.