L'enrobage moléculaire: complexation du cation sodium par le dibenzo-24-couronne-8

Structure and microdynamics in solution of the dibenzo-24-crown-8 – sodium perchlorate complex have been studied by Na-23 and C-13 nmr. In all solvents (acetone, acetonitrile, pyridine, nitromethane, propylene carbonate, and chloroform-d) and in the binary mixtures acetone/pyridine, acetonitrile/nitromethane, propylene carbonate/methanol, the crown wraps around the sodium cation and expels the conjugate anion and the solvent molecules from the cation coordination sphere. Measurements of Na-23 quadrupolar and C-13 dipole–dipole relaxation times give the effective correlation time of the complex in CDCl3 solution (65 ± 11 ps) and the Na-23 quadrupolar coupling constant in all those solvents and binary mixtures (0.8–1.2 MHz). The sodium coordination shell is completely frozen during a time superior to the reorientational correlation time.