CHEMICAL AND HEISENBERG EXCHANGE IN ION-PAIR SOLUTIONS REVISITED BY 2-DIMENSIONAL EPR SPECTROSCOPY

The dynamic processes in tetrahydrofuran (THF) solutions of 2,5-di-ter t-butyl-p-benzoquinone(-), Na+ (DtBPBQ(-), Na+) ion-pairs, obtained by reduction with a sodium mirror, have been studied by two-dimensional (2D) exchange Fourier transform (FT) EPR spectroscopy. Measurements we re made at room temperature (17-20 degrees C) on solutions with radica l concentrations ranging from 4.5 X 10(-5) to 3.5 x 10(-3) M and with mixing times varying from 0.3 to 6 mu s. Analysis of the EPR spectra i ndicates the presence of two types of DtBPBQ(-), Na+ ion-pairs, which are labeled A and B. In both species there is intramolecular Na+ hoppi ng, but while in ion-pair A the process is slow and suitable for monit oring by the 2D exchange method, in ion-pair B it is much faster and r esults in a selective smearing of some of the hyperfine lines. The ori gin of the latter is tentatively ascribed to complexation with OH- gen erated by water impurity. In the 2D spectra characteristic cross peaks due to Na+ hopping and Heisenberg exchange (HE) appear. Analysis of t hese spectra provides information about the intramolecular Na+ hopping rate in ion-pair A, k(CE)(A), as well as on the HE rate constants, k( ii), of the various radicals. At room temperature these are k(CE)(A) = (1.7 +/- 0.2) x 10(5) s(-1), k(AA) = (7.5 +/- 1.0) X 10(8) s(-1) mol( -1), and k(BB) = (1.0 +/- 0.2) X 10(8) s(-1) mol(-1). These results al so provide information on the longitudinal relaxation rates of the ove rall magnetization of both radicals. These depend on the total radical concentration and within experimental accuracy are the same for both radicals. The mechanism for this process is tentatively ascribed to el ectron-electron (radical-radical) dipolar interaction, and its rate is compared with calculations based on the point dipole approximation. T he present work demonstrates the power of the 2D exchange EPR method i n elucidating mechanisms of dynamic processes and determining kinetic parameters, in particular when several such processes occur simultaneo usly.