Relaxation Mechanisms

For nmr spectroscopy to be practical, an efficient mechanism for nuclei in the higher energy ^_1/2 spin state to return to the lower energy +1/2 state must exist. In other words, the spin population imbalance existing at equilibrium must be restored if spectroscopic observations are to continue. Now an isolated spinning nucleus will not spontaneously change its spin state in the absence of external perturbation. Indeed, hydrogen gas (H₂) exists as two stable spin isomers: ortho (parallel proton spins) and para (antiparallel spins). Nmr spectroscopy is normally carried out in a liquid phase (solution or neat) so that there is close contact of sample molecules with a rapidly shifting crowd of other molecules (Brownian motion). This thermal motion of atoms and molecules generates local fluctuating electromagnetic fields, having components that match the Larmor frequency of the nucleus being studied. These local fields stimulate emission/absorption events that establish spin equilibrium, the excess spin energy being detected as it is released. This relaxation mechanism is called Spin-Lattice Relaxation (or Longitudinal Relaxation). The efficiency of spin-lattice relaxation depends on factors that influence molecular movement in the lattice, such as viscosity and temperature. The relaxation process is kinetically first order, and the reciprocal of the rate constant is a characteristic variable designated T₁, the spin-lattice relaxation time. In non-viscous liquids at room temperature T₁ ranges from 0.1 to20 sec. A larger T₁ indicates a slower or more inefficient spin relaxation.
Another relaxation mechanism called spin-spin relaxation (or transverse relaxation) is characterized by a relaxation time T₂. This process, which is actually a spin exchange, will not be discussed here.