Relaxation Time of NMR

Relaxation Time of NMR

What is relaxation time?

Relaxation time, the time required to reach thermal equilibrium. is a characteristic time of the dynamical system. The time required for a certain variable of the system to go from a transient state to a certain steady state. In statistical mechanics and thermodynamics, relaxation time represents the time required for a system to go from an unstable steady state to a stable steady state.

What is NMR relaxation time?

First of all, you need to understand the basic principle of NMR: it can be understood as the resonance of atomic nuclei in a magnetic field. The nucleus in general NMR refers to the hydrogen nucleus. Magnetism refers to the magnetic field environment. In a balanced and stable magnetic field, hydrogen nuclei will precess at a fixed frequency, and the precession frequency is proportional to the strength of the magnetic field. Resonance means that when the external frequency is equal to the natural frequency of hydrogen nuclei in a magnetic field, the hydrogen nuclei absorb energy and undergo nuclear magnetic resonance.

The process of NMR

The process of NMR is actually the process of atomic nuclei absorbing radio frequency energy. When the radio frequency pulse is turned off, the energy-absorbing atomic nuclei will release the absorbed energy. After a certain relaxation process, as time goes by, they will eventually return to an equilibrium state. The time required for the nucleus to release energy corresponds to the NMR relaxation time.

Two types of NMR relaxation time,T1 and T2

T1 is the longitudinal relaxation time, and the time constant T1 of longitudinal magnetization recovery is called longitudinal relaxation time (also known as spin-lattice relaxation time).

T2 is the transverse relaxation time, and the time constant T2 for the disappearance of transverse magnetization is called the transverse relaxation time (also known as spin-spin relaxation time).

 NMR relaxation time is related to:

NMR relaxation time T1:

The relaxation process is the process of energy release. The spin lattice is equivalent to the lattice composed of H atoms, so the energy is released into the surrounding lattice. T1 relaxation is closely related to the motion of surrounding molecules. T1 can study slow molecular motion, such as the chelation state of metal ions, protein aggregation, surface dynamics of porous materials, etc.

NMR relaxation time T2: 

T2, spin-spin relaxation. Because the Larmor frequency (or phase) of each H proton is not the same, when the radio frequency pulse is removed, the process of protons from gathering to scattering.

Factors affecting NMR relaxation time T2:

• Internal factors

Molecular motion: the slower the molecular motion, the smaller the T2; such as ice and solid;

Molecular size: The larger the molecular size, the smaller the T2; for example, the relaxation time of macromolecules such as starch in food is much shorter than that of water and oil.

Molecular binding state: the tighter the binding, the smaller the T2; the multilayer structure theory of water in food.

• External factors

Magnetic field inhomogeneity: The magnetic field inhomogeneity will accelerate the dephasing process (making the difference between H protons larger), so that the measured T2 decays much faster than the actual T2.