Pulsed microwave irradiation effects on the spin dynamics of radical pairs in micellar solutions

In the reactions between multiplet molecules, there is a strict rule of conservation of the total spin multiplicity between starting materials and products. In radical pair reactions, the combination of two doublet species forms one singlet and three triplet sublevels. Figure 1 shows a vector diagram of the spin states of a radical pair and their abbreviates in the presence of magnetic fields. Among these four states, only the singlet one (S) can recombine to form a singlet product. Since S and the triplet T0 states are degenerate, the hyperfine interaction (hfc) and the difference in Larmor precession frequency (∆gβB) induces intersystem crossing between them. Practically, they behave as one mixed state. On the contrary, the T+1 and T−1 states are energetically separated from the S and T0 states by the Zeeman interaction, which hinders the conversion of these states. The slow relaxation between them is the only path connecting these states to the mixed state. Consequently, the radical pairs in the T0 have a chance to recombine but those in the T+1 and T−1 states dissociate forming escaping radicals. This is the origin of the external magnetic field effect of radical pair reactions.