where 𝑓 𝑀 is the mole fraction of bound water molecules (with protons at distance r from the paramagnetic moiety), 𝜔 𝑆 is the electron Larmor frequency, 𝜏 𝑅 , 𝜏 𝑒 and 𝜏 𝑀 are the reorientation time, electron relaxation time and residence time, respectively, Δ 𝑡 2 is the mean squared fluctuation of the zero-field splitting (for S > 1/2) and 𝜏 𝑣 is the correlation time for the instantaneous distortions of the coordination polyhedron of the paramagnetic metal.