EPR SPECTROSCOPY OF ELECTRON-SPIN POLARIZED BIRADICALS IN LIQUID SOLUTIONS - TECHNIQUE, SPECTRAL SIMULATION, SCOPE, AND LIMITATIONS

Application of the time-resolved EPR technique to the study of unstable organic biradicals in liquid solution is described. Results for a homologous series of polymethylene chain biradicals, produced by laser flash photolysis of cyclic ketones, are presented as experimental examples. To account for the shape and decay of the spectra with time, two models for the EPR spectra are put forward. The first uses a perturbation theory approximation for the eigenvectors, eigenvalues, and populations of the states, and then applies kinetic decay rate constants for nuclear spin dependent chemical reaction and spin relaxation. The second method shows how symmetry factoring the nuclear basis allows the block diagonalization of the spin Hamiltonian, so that the eigenvalue problem, even for 16 hyperfine interactions, can be solved exactly. A comparison of the two models shows that the perturbation treatment is adequate for most cases. The shape and time dependence of the EPR spectra for both acyl-alkyl and bis(alkyl) polymethylene biradicals are simulated and good agreement with experiment is obtained. The electronic spin-spin coupling interaction, J, and the end-to-end contact rate of the chains, k(r), are extracted from the simulations for each chain length. The lifetimes are 1-2-mu-s for the bis(alkyl) biradicals and about 0.5-mu-s for the acyl-alkyl species.