EFFECT OF IONOPHORES ON THE RATE OF INTRA-MOLECULAR CATION-EXCHANGE IN DUROSEMIQUINONE ION-PAIRS

The ionophores 15-crown-5 (15C5), 18-crown-6 (18C6), dibenzo-18-crown-6 (DBC), and cryptand-222 (C222) have been used to form ionophore complexed ion pairs in benzene with the sodium salt of the durosemiquinone anion radical (Na+DQ-). The electron paramagnetic resonance (EPR) spectra of the 18C6 and 15C5 complexes with Na+DQ- show an alternating line width effect which indicates that the sodium ion is undergoing intramolecular exchange between two equivalent sites, one near each oxygen atom. An analysis of the rate of this exchange between about 280 and 330 K by means of the modified Bloch equations yields the Arrhenius parameters Ea = 8.7 ± 0.3 kcal/mol and A = (9 ± 3) × 1012 s-1 for the 18C6 complex and Ea = 6.0 ± 0.2 kcal/mol and A ∼ 1012 s-1 for the 15C5 complex. The preexponential factor (A) in both cases is in the range 1012-1013 s-1. The EPR spectra obtained from the DEC complex between 281 and 333 K showed no evidence for the exchange of the crown complexed Na+, while the spectra for the C222 complex appeared to indicate fast exchange. The hyperfine splitting constants measured in this work showed little temperature variation. The experimental results indicate that the structures of the ionophore complexed ion pairs vary little with temperature and that the kinetic parameters for the exchange reaction in benzene are primarily dependent on the identity of the ionophore. © 1979 American Chemical Society.