Effect of isotopic substitution on association with the sodium counterion

EPR studies have shown that in tetrahydrofuran the associative interaction 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 ofr the naphthalene anion radical is increased upon perdeuteriation of the naphthalene. This may, in part, be due to a perturbation of the vibrational energies of the anion, but thermal dependence seems to indicate the the equilibrium constant controlling the dissociation of the tight ion pair to the solvent-separated ion pair (Na[sup +],C[sub 10]H[sub 8][sup [center dot]][sup [minus]] = Na[sup +]//C[sub 10]H[sub 8][sup [center dot]][sup [minus]]) is also affected (K(perprotiated)/K(perdeuteriated) = 0.974 [plus minus] 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 to the large gyromagnetic ratio fo the sodium nucleus, this system is ideal for observation of this effect.