Chapter 3: Heteronuclear Cross-relaxation

Heteronuclear cross-relaxation is the dynamical process by which two distinct nuclear spins interact through space by a dipole–dipole mechanism, modulated by the re-orientational correlation time of their connection vector. Because of its inverse power six with the nuclei separation, it is a short distance effect (generally no more than 3 Å). In the intramolecular field, it appears as a very useful auxiliary means for structural analysis. In intermolecular effects, it is one of the most important keys for probing close approaches in solute–solvent interactions. A great emphasis is given here in this topic of solution chemistry with numerous examples in general chemistry (ionic liquids, ion pairs, aggregates) and in biology (hydration of biomolecules). In addition, all the relevant sequences used to measure this interaction are examined, as per their advantages and disadvantages. Finally, a theoretical insight in intermolecular effects is discussed, as per the so-called ‘force-free hard sphere diffusion model’ to extract both the right dependence with the relative diffusion of the species in solution and their separation distance to eradicate some wrong common misinterpretations.