where NA is the Avogadro's constant, [M] is the concentration of the paramagnetic centers, expressed in mol dm -3 , 𝐷 1 and 𝐷 2 equal to the diffusion coefficient of the water molecule and of the molecule bearing the paramagnetic moiety (usually negligible) and 𝜏 𝐷 = 𝑑 2 /(𝐷 1 + 𝐷 2 ).