EFFECT OF PI-SYSTEM SPACERS ON EXCHANGE COUPLINGS AND END-TO-END ENCOUNTER RATES IN FLEXIBLE BIRADICALS

Time-resolved electron paramagnetic resonance (EPR) studies of flexible biradicals containing meta- and para-linked aromatic spacers (aryl ethers) between the unpaired spins are reported. Spectral simulation allows determination of the scalar electronic spin-spin coupling J and the end-to-end encounter rate k(en) between the two radical centers. The biradicals have different J couplings compared to saturated polymethylene chain biradicals of similar length. The value of k(en) obtained for the para isomer was an order of magnitude lower than that of the saturated system, while for the meta isomer it was comparable. Both J and k(en) show large changes with temperature for the para isomer (50-degrees-C vs 105-degrees-C). The results are discussed in terms of through-bond (TB) and through-solvent (TS) coupling mechanisms. Semiempirical MO calculations of overlap integrals support the hypothesis that the TB component should be weaker for the meta isomer. It is concluded that the TS component of the J coupling is dominant for both biradicals at these elevated temperatures.