This tutorial review deals with the methodological procedures that can be used to obtain accurate molecular sizes in solution from diffusion NMR spectroscopy. The critical aspects associated with the estimation of the size of molecules from the measured translational self-diffusion coefficient, using the Stokes–Einstein equation, are highlighted and discussed. From a theoretical point of view, it is shown how to take into account the size of the solute with respect to that of the solvent and its non-spherical shape using the appropriate correction factors in the frictional coefficient. From a practical point of view, the advantages of introducing an internal standard in the sample are presented. Initially, non-aggregating systems are considered in an attempt to clarify what hydrodynamic dimensions mean. Successively, aggregating systems are addressed showing how it is possible to understand the aggregation level and derive the thermodynamic parameters for some illustrative aggregation processes.

We report on three Ir(III) molecular catalysts for water oxidation: 1, [Cp*Ir(ppy)Cl]; 2, [Cp*Ir(bzpy)NO₃]; 3, [Cp*Ir(H₂O)₃](NO₃)₂. 2 and 3 are water-soluble and show a long-term activity ca. 2 and 3 times higher than 1. It is remarkable that 3, having the simplest structure, is the catalyst with the highest activity.

The activity of three [Cp*IrL_n] (Cp* = pentamethylcyclopentadienyl) archetypal catalysts ([Cp*Ir (bpy)Cl]Cl (1, bpy = 2,2′-bipyridine), [Cp*Ir(bzpy)(NO₃)] (2, bzpy = 2-benzoylpyridine) and [Cp*Ir(H₂O)₃](NO₃)₂ (3)) for water oxidation to molecular oxygen was compared using cerium(IV) ammonium nitrate as a sacrificial oxidant. Kinetic studies were carried out by: i) measuring the depletion of Ce⁴⁺ through UV-Vis spectroscopy, ii) directly detecting the evolved oxygen through the Clark electrode and iii) measuring the volume of the evolved oxygen. The kinetics of Ce⁴⁺ consumption were zero-order in Ce⁴⁺ for catalysts 2 and 3, while they were first-order for 1. The order with respect to catalyst was 1 for 1 and 2 while it was 1.5 for 3. As a consequence, 2 (TOF_max = 14.4 min⁻¹) and 3 (TOF_max = 50.4 min⁻¹) were found to be the most active catalysts at low and high catalyst concentration, respectively, while the performance of 1 (TOF_max = 8.6 min⁻¹) increased with increasing the concentration of Ce⁴⁺. 1 and 3 were found to be the most robust catalysts at low (3.1 μM, TON = 1240) and high (7.0 μM, TON = 4042) catalyst concentration, respectively. In situNMR studies were performed under exactly the same conditions of the catalytic experiments. It was observed that Cp* underwent an oxidative degradation, ultimately leading to acetic, formic and glycolic acids. Several Ir-containing intermediates of the degradation process were intercepted and fully characterized in solution through 1D- and 2D-NMR experiments. DFT and NMR studies indicated that the degradation proceeds via an initial double oxidative functionalization of both the quanternary carbon and proton of a Cp* C–CH₃ moiety.