INFLUENCE OF MOLECULAR-STRUCTURE ON THE RATE OF INTERSYSTEM CROSSING IN FLEXIBLE BIRADICALS

Laser flash photolysis (LFP), low-field chemically induced dynamic nuc lear polarization (CIDNP), and time-resolved electron paramagnetic res onance (TREPR) techniques have been used for the comparative study of magnetic field and spin effects in acyl-ketyl and bis(ketyl)biradicals formed during the photolysis of 2-hydroxy-2,12-dimethylcyclododecanon e (1-OH) and 2,12-dihydroxy-2,12-dimethylcyclododecanone (2-OH), respe ctively. The short biradical lifetime, the small magnetic field effect (MFE) on a biradical lifetime, and the low intensity of spin-correlat ed radical pair (SCRP) polarization observed during the photolysis of 1-OH indicate that the main channel of intersystem crossing in acyl-ke tyl biradical is spin-orbit coupling (SOC). For bis(ketyl) biradicals observed during the photolysis of 2-OH, SOC is of minor importance, an d both MFE and SCRP polarization are much larger. It is shown that the presence of significant SOC in biradicals can result in an increase o f the CIDNP intensity at low magnetic fields. Calculations of biradica l evolution, based on the numerical solution of the stochastic Liouvil le equation (SLE), were carried out by taking into account (1) the mol ecular dynamics of the polymethylene chain linking the radical centers , (2) the distance-dependent exchange interaction, (3) the chemical re actions of the biradical, (4) the spin relaxation processes, and (5) s tate-to-state transitions caused by hyperfine interaction and SOC. A c ommon set of parameters was found which allows quantitative descriptio n of the field dependence of the biradical lifetime, the CIDNP field d ependence, and the TREPR spectra and kinetics.