Effect of Localized Unsaturation on the Scalar Exchange Coupling in Flexible Biradicals

Article abstract of DOI:10.1021/j100115a051

Time-resolved electron paramagnetic resonance (TREPR) spectra of flexible biradicals containing one and two double bonds are reported.Simulation of the strongly spin-polarized spectra allows determination of the scalar electronic exchange coupling J and the encounter rate k_en between the two radical centers.Comparison of each unsaturated biradical with the saturated analogue of similar chain length shows that the J coupling increases with the double bonds present.This and other effects, such as that of the solvent on saturated biradical EPR spectra, are modeled and discussed in terms of both through-bond (TB) and through-solvent (TS) coupling mechanisms.Monte Carlo simulations of the chain dynamics and semiempirical molecular orbital calculations of overlap integrals support the hypothesis that isolated trans double bonds decrease the TS component of the J coupling while increasing the TB component.