Diffusion NMR in Supramolecular Chemistry

Yoram Cohen, Liat Avram, Tamar Evan-Salem and Limor Frish 

Several different analytical methods are needed to determine the structure and dynamics and to map the intermolecular interactions that prevail in supramolecular systems in solution [1]. NMR is one of the most powerful of these methods [Id-e]. The conventional NMR parameters that are used to characterize the structure and dynamics of supramolecular systems in solution are chemical shifts, spin-spin coupling, relaxation times, NOEs (Nuclear Overhauser Effect) and the correlation thereof. We shall demonstrate that the diffusion coefficient, which is currently underused, should be added to this arsenal of NMR parameters when characterizing supramolecular systems in solution. 

In the present chapter, after a brief description of the notion of diffusion and the basics of diffusion NMR, we briefly survey a few technical issues concerning diffusion NMR. Readers interested in more detailed descriptions of the theory of diffusion NMR can consult the many excellent reviews published in recent years [5-7]. 

We shall then describe selected applications of diffusion NMR that will form the main body of the present chapter. Thereafter, we shall provide a brief discussion on limitations of diffusion NMR and the effect of chemical exchange on diffusion NMR. We conclude with a short summary and outlook. 

Self-diffusion is the random translational motion of ensembles of particles (molecules or ions) originating from their thermal energy. It is well known that diffusion, which is closely related to the molecular size of the diffusing species, is given by the Einstein-Smoluchowski equation, Eq. (6.1) [8]  

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Cohen, Y., Avram, L., Evan-Salem, T., Frish, L., Diffusion NMR in supramolecular chemistry , in Analytical Methods in Supramolecular Chemistry, Schalley, C. A., ed. Wiley-VCH Weinheim, 2007, pp. 163-219. 

Applications of Diffusion NMR in Supramolecular Chemistry Selected Examples... 

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