Diffusion Ordered NMR Spectroscopy (DOSY)

Diffusion NMR spectroscopy has become an essential tool for investigating the supramolecular assembling processes that from molecular “bricks” lead to the construction of functional nanomaterials and nano-sized catalysts. This is probably due to the implementation and commercialization of new NMR instrumentations with the default capability of generating pulsed-field gradients (PFGs) along the direction of the magnetic field. Furthermore, while a robust package of analytical techniques is available to investigate molecules and extended materials or large biomolecules, which are the two-dimensional extremes, the characterization of the chemical mesoscale (several nanometers) is particularly challenging. It is just in this context, that is, the characterization of objects with an intermediate dimen- sion ranging from dozens of angstroms to hundreds of nanometers, that diffusion NMR spectroscopy shows all its potentialities. The aim of this chapter is not to discuss in detail the underlying NMR pulse sequences of diffusion experiments. The basic methodology is longstanding and excellent reviews have already been published. Here, we want to discuss diffusion NMR experiments from a pragmatic point of view in order to show what information can be obtained and how reliable it is, focusing attention on supramolecular objects of “intermediate” dimensions. In particular, after recalling the principles underlying diffusion NMR spectroscopy and the measure- ment of the translational self-diffusion coefficient (Dt) (Section 2), we show how accurate hydrodynamic dimensions can be derived from Dt once the shape and size of the diffusing particles have been correctly taken into account (Section 3). Later on, the application of diffusion NMR to the study of supramolecular systems is described (Section 4) in terms of determination of the average hydrodynamic dimensions and thermodynamic parameters of the self-assembly processe