Figure 6. Nuclear relaxation.

A) Longitudinal Relaxation. After an rf-pulse of energy is applied to the nuclei in solution, the atoms must relax back to their equilibrium position in line with the z-axis by releasing energy in the form of heat to its surrounding. B) Transverse Relaxation. After an rf-pulse of energy is applied to the nuclei in solution, the signals corresponding to the different nuclei fan out away from the x-axis at different rates due to chemical shift dispersion and diffusion and the transfer of energy from one spin to another spin, which proceeds until the net magnetization becomes zero. For small molecules the rate of transverse relaxation often equals the rate of longitudinal relaxation because there lacks sufficient interaction between the atoms to permit the transfer of excitation energy. For larger molecules, especially proteins, the rate of transverse relaxation is generally faster than longitudinal relaxation because the transfer of energy between spins occurs much more quickly and efficiently than the transfer of energy to the surroundings.