In the case that the 𝜏 𝐷 /𝜏 𝑒 term can be neglected, at low fields this three-dimensional translational diffusion depends on the square root of the frequency, as shown in Fig. , with a coefficient related to the diffusion coefficients. The same dependence is not apparent when the electron relaxation also contributes to modulate the electron-nucleus dipole-dipole interaction. . Field dependence of the water proton relaxation rate according to the outer-sphere relaxation model (Eqs. 9, 11) for water molecules diffusing, up to a distance of closest approach of 3.6 Å, around spherical molecules containing a Gd 3+ ion (S=7/2, in 1 mM concentration), with diffusion coefficient 𝐷 1 + 𝐷 2 of 2.4×10 -9 m 2 /s and electron relaxation described by the parameters ∆ 𝑡 = 3.77×10 9 s -1 (corresponding to 0.02 cm -1 ) and (i) 𝜏 𝑣 = 20 ps (solid lines) or (ii) 0.2 ps (dotted lines). In the second case, the dipole-dipole interaction is modulated only by the diffusional time 𝜏 𝐷 ( 𝜏 𝐷 𝜏 𝑒 ≪ 1). In the right panel, the rates are plotted against the square root of the frequency to show its linear dependence at low fields in the case that electron relaxation is negligible.