EFFECT OF ISOTOPIC-SUBSTITUTION ON ASSOCIATION WITH THE SODIUM COUNTERION

EPR studies have shown that in tetrahydrofuran the associative interac tion between the sodium cation and an organic anion is perturbed upon replacement of the protons in the anion with deuteriums. Specifically, the sodium hyperfine coupling constant for the naphthalene anion radi cal is increased upon perdeuteriation of the naphthalene. This may, in part, be due to a perturbation of the vibrational energies of the ani on, but thermal dependence seems to indicate that the equilibrium cons tant controlling the dissociation of the tight ion pair to the solvent -separated ion pair (Na+,C10H8.- = Na+//C10H8.-) is also affected (K(p erprotiated)/K(perdeuteriated) = 0.974 +/- 0.005 at 298 K). The van't Hoff plots show that the enthalpy of this dissociation may be as large as 120 J/mol more exothermic for the isotopically light system. Due t o the large gyromagnetic ratio of the sodium nucleus, this system is i deal for observation of this effect.