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.

The activity of three [Cp*IrL(n)] (Cp* = pentamethylcyclopentadienyl) archetypal catalysts ([Cp*Ir (bpy)Cl]Cl (1, bpy = 2,2'-bipyridine), [Cp*Ir(bzpy)(NO(3))] (2, bzpy = 2-benzoylpyridine) and [Cp*Ir(H(2)O)(3)](NO(3))(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 Ce(4+) 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 Ce(4+) consumption were zero-order in Ce(4+) 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 mu M, TON = 1240) and high (7.0 mu M, TON = 4042) catalyst concentration, respectively. In situ NMR 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-CH(3) moiety

We definitively show that the CO stretching response to metal coordination is driven exclusively by π polarization, which quantitatively correlates with π back-donation and changes in CO bond length and frequency.

The utility of PGSE NMR measurements in determining hydrodynamic radii (r H) and volumes (V H) of small- and medium-size molecules (3 Å < r H < 6 Å) was evaluated by performing measurements for a variety of pure deuterated solvents and their solutions containing the internal standard TMSS [tetrakis(trimethylsilyl)silane] also in the presence of a variable concentration of 3BPh4. It was found that accurate r H and V H values can be obtained by introducing in the Stokes−Einstein equation (D t = kT/cπηr H) not only the correct values for temperature (T) and viscosity (η) but, particularly, that for the c factor. PGSE NMR measurements were then applied to an investigation of the aggregation tendency of complexes [Ru(η6-cymene)(R1R2NCH2CH2NR1R2)Cl]X (R1 = R2 = H, 1; R1 = H, R2 = H, 2; R1 = R2 = Me, 3; X- = PF6 - or BPh4 -) in both protic and aprotic solvents with a relative permittivity (εr) ranging from 4.81 (chloroform-d) to 32.66 (methanol-d 4). Compounds 1 and 2 exhibited a remarkable tendency to aggregate through intercationic N−H···Cl and cation/anion N−H···FPF5 - hydrogen bonds. In addition to ion pairs, ion triples and ion quadruples were also observed in solution. Compound 3, having no N-H moiety, showed less tendency to aggregate than 1 and 2, even though it also afforded ion quadruples in apolar and aprotic solvents. Relative anion−cation orientations and arene conformations were investigated by means of 1H−NOESY and 19F,1H-HOESY NMR spectroscopy. The relative anion−cation position was well-defined, especially for compounds bearing the PF6 - counterion, and was modulated by the nature of the N,N ligand. A progressive slackening of the contact aggregates was observed in the series 1−3 that led to a higher mobility of the anion, as indicated by the observation of less specific interionic NOEs.