Theoretical study on the intersystem crossing mechanism of a diradical in Norrish type II reactions in solution

We studied the theoretical mechanism of intersystem crossing (ISC) of a diradical involved in Norrish type II reactions in solution, on the basis of ab initio molecular orbital calculations and molecular dynamics (MD) simulation. The ab initio calculations were performed for the simple 1,4-diradical C₄H₈O to obtain the potential energy surfaces and the spin-orbit coupling (SOC) elements. These results revealed that the energy splitting and the SOC element between the singlet and triplet states vary with the solute geometry in a complicated manner and have no correlation to the radical site distance, indicating that the through-bond interaction is dominant to characterize these properties. We constructed the analytical functions representing the potential energy surfaces, the energy splitting, and the SOC element by considering the orbital interaction. Further the MD calculations were carried out for the diradical in methanol solvent, and the rate constant of ISC driven by SOC was calculated from the MD trajectories for each solute isomer. By analyzing the rate with microscopic dynamics, we found that localized transition is not realized for some conformers, but the transition state theory holds rather good to estimate the rate. The deviation does not come from the recrossing effect as commonly discussed but from the fact that the coupling element varys with time during the transition. The solvent effect on the ISC rate was extensively discussed from both the dynamic and static viewpoints.